diff --git a/smart-contracts/contracts/mixins/MixinConvenienceOwnable.sol b/smart-contracts/contracts/mixins/MixinConvenienceOwnable.sol
index bc749c5a2b4..4c6e42c16ee 100644
--- a/smart-contracts/contracts/mixins/MixinConvenienceOwnable.sol
+++ b/smart-contracts/contracts/mixins/MixinConvenienceOwnable.sol
@@ -2,13 +2,14 @@
 pragma solidity ^0.8.0;
 
 import './MixinLockCore.sol';
+import './MixinErrors.sol';
 
 /**
  * @title Mixin to add support for `ownable()`
  * @dev `Mixins` are a design pattern seen in the 0x contracts.  It simply
  * separates logically groupings of code to ease readability.
  */
-contract MixinConvenienceOwnable is MixinLockCore {
+contract MixinConvenienceOwnable is MixinErrors, MixinLockCore {
 
   // used for `owner()`convenience helper
   address private _convenienceOwner;
@@ -38,7 +39,9 @@ contract MixinConvenienceOwnable is MixinLockCore {
    */ 
   function setOwner(address account) public {
     _onlyLockManager();
-    require(account != address(0), 'OWNER_CANT_BE_ADDRESS_ZERO');
+    if(account == address(0)) {
+      revert OWNER_CANT_BE_ADDRESS_ZERO(); 
+    }
     address _previousOwner = _convenienceOwner;
     _convenienceOwner = account;
     emit OwnershipTransferred(_previousOwner, account);
diff --git a/smart-contracts/contracts/mixins/MixinERC721Enumerable.sol b/smart-contracts/contracts/mixins/MixinERC721Enumerable.sol
index 808ac4b778b..1f04cee3074 100644
--- a/smart-contracts/contracts/mixins/MixinERC721Enumerable.sol
+++ b/smart-contracts/contracts/mixins/MixinERC721Enumerable.sol
@@ -3,7 +3,7 @@ pragma solidity ^0.8.0;
 
 import './MixinKeys.sol';
 import './MixinLockCore.sol';
-// import '@openzeppelin/contracts-upgradeable/token/ERC721/extensions/IERC721EnumerableUpgradeable.sol';
+import './MixinErrors.sol';
 import '@openzeppelin/contracts-upgradeable/utils/introspection/ERC165StorageUpgradeable.sol';
 
 
@@ -12,6 +12,7 @@ import '@openzeppelin/contracts-upgradeable/utils/introspection/ERC165StorageUpg
  */
 contract MixinERC721Enumerable is
   ERC165StorageUpgradeable,
+  MixinErrors,
   MixinLockCore, // Implements totalSupply
   MixinKeys
 {
@@ -37,7 +38,9 @@ contract MixinERC721Enumerable is
   ) public view
     returns (uint256)
   {
-    require(_index < _totalSupply, 'OUT_OF_RANGE');
+    if(_index >= _totalSupply) {
+      revert OUT_OF_RANGE();
+    }
     return _index;
   }
 
diff --git a/smart-contracts/contracts/mixins/MixinErrors.sol b/smart-contracts/contracts/mixins/MixinErrors.sol
new file mode 100644
index 00000000000..aea2842b464
--- /dev/null
+++ b/smart-contracts/contracts/mixins/MixinErrors.sol
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+/**
+ * @title List of all error messages 
+ * (replace string errors message to save on contract size)
+ */
+contract MixinErrors {
+  
+  // generic
+  error OUT_OF_RANGE();
+  error NULL_VALUE();
+  error INVALID_ADDRESS();
+  error INVALID_TOKEN();
+  error INVALID_LENGTH();
+  error UNAUTHORIZED();
+
+  // erc 721
+  error NON_COMPLIANT_ERC721_RECEIVER();
+
+  // roles
+  error ONLY_LOCK_MANAGER_OR_KEY_GRANTER();
+  error ONLY_KEY_MANAGER_OR_APPROVED();
+  error UNAUTHORIZED_KEY_MANAGER_UPDATE();
+  error ONLY_LOCK_MANAGER_OR_BENEFICIARY();
+  error ONLY_LOCK_MANAGER();
+
+  // single key status
+  error KEY_NOT_VALID();
+  error NO_SUCH_KEY();
+
+  // single key operations
+  error CANT_EXTEND_NON_EXPIRING_KEY();
+  error NOT_ENOUGH_TIME();
+  error NOT_ENOUGH_FUNDS();
+
+  // migration & data schema
+  error SCHEMA_VERSION_NOT_CORRECT();
+  error MIGRATION_REQUIRED();
+
+  // lock status/settings
+  error OWNER_CANT_BE_ADDRESS_ZERO();
+  error MAX_KEYS_REACHED();
+  error KEY_TRANSFERS_DISABLED();
+  error CANT_BE_SMALLER_THAN_SUPPLY();
+
+  // transfers and approvals
+  error TRANSFER_TO_SELF();
+  error CANNOT_APPROVE_SELF();
+
+  // keys management 
+  error LOCK_SOLD_OUT();
+
+  // purchase
+  error INSUFFICIENT_ERC20_VALUE();
+  error INSUFFICIENT_VALUE();
+
+  // renewals
+  error NON_RENEWABLE_LOCK();
+  error LOCK_HAS_CHANGED();
+  error NOT_READY_FOR_RENEWAL();
+
+  // gas refund
+  error GAS_REFUND_FAILED();
+
+  // hooks
+  // NB: `hookIndex` designed the index of hook address in the params of `setEventHooks`
+  error INVALID_HOOK(uint8 hookIndex);
+
+}
diff --git a/smart-contracts/contracts/mixins/MixinFunds.sol b/smart-contracts/contracts/mixins/MixinFunds.sol
index 57c87970d11..7b8fc92ad8f 100644
--- a/smart-contracts/contracts/mixins/MixinFunds.sol
+++ b/smart-contracts/contracts/mixins/MixinFunds.sol
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: MIT
 pragma solidity ^0.8.0;
 
+import './MixinErrors.sol';
 import '@openzeppelin/contracts-upgradeable/token/ERC20/IERC20Upgradeable.sol';
 import '@openzeppelin/contracts-upgradeable/utils/AddressUpgradeable.sol';
 import '@openzeppelin/contracts-upgradeable/token/ERC20/utils/SafeERC20Upgradeable.sol';
@@ -9,7 +10,7 @@ import '@openzeppelin/contracts-upgradeable/token/ERC20/utils/SafeERC20Upgradeab
  * @title An implementation of the money related functions.
  * @author HardlyDifficult (unlock-protocol.com)
  */
-contract MixinFunds
+contract MixinFunds is MixinErrors
 {
   using AddressUpgradeable for address payable;
   using SafeERC20Upgradeable for IERC20Upgradeable;
@@ -34,10 +35,13 @@ contract MixinFunds
   internal 
   view
   {
-    require(
-      _tokenAddress == address(0) || IERC20Upgradeable(_tokenAddress).totalSupply() > 0,
-      'INVALID_TOKEN'
-    );
+    if(
+      _tokenAddress != address(0) 
+      && 
+      IERC20Upgradeable(_tokenAddress).totalSupply() < 0
+    ) {
+      revert INVALID_TOKEN();
+    }
   }
 
   /**
diff --git a/smart-contracts/contracts/mixins/MixinGrantKeys.sol b/smart-contracts/contracts/mixins/MixinGrantKeys.sol
index c471270f5ce..a8105fc9c02 100644
--- a/smart-contracts/contracts/mixins/MixinGrantKeys.sol
+++ b/smart-contracts/contracts/mixins/MixinGrantKeys.sol
@@ -3,6 +3,7 @@ pragma solidity ^0.8.0;
 
 import './MixinKeys.sol';
 import './MixinRoles.sol';
+import './MixinErrors.sol';
 
 
 /**
@@ -12,6 +13,7 @@ import './MixinRoles.sol';
  * separates logically groupings of code to ease readability.
  */
 contract MixinGrantKeys is
+  MixinErrors,
   MixinRoles,
   MixinKeys
 {
@@ -25,10 +27,14 @@ contract MixinGrantKeys is
     address[] calldata _keyManagers
   ) external {
     _lockIsUpToDate();
-    require(isKeyGranter(msg.sender) || isLockManager(msg.sender), 'ONLY_LOCK_MANAGER_OR_KEY_GRANTER');
+    if(!isKeyGranter(msg.sender) && !isLockManager(msg.sender)) {
+      revert ONLY_LOCK_MANAGER_OR_KEY_GRANTER();
+    }
 
     for(uint i = 0; i < _recipients.length; i++) {
-      require(_recipients[i] != address(0), 'INVALID_ADDRESS');
+      if(_recipients[i] == address(0)) { 
+        revert INVALID_ADDRESS();
+      }
 
       // an event is triggered
       _createNewKey(
diff --git a/smart-contracts/contracts/mixins/MixinKeys.sol b/smart-contracts/contracts/mixins/MixinKeys.sol
index cbd5e72e681..7bc278b49be 100644
--- a/smart-contracts/contracts/mixins/MixinKeys.sol
+++ b/smart-contracts/contracts/mixins/MixinKeys.sol
@@ -2,6 +2,7 @@
 pragma solidity ^0.8.0;
 
 import './MixinLockCore.sol';
+import './MixinErrors.sol';
 
 /**
  * @title Mixin for managing `Key` data, as well as the * Approval related functions needed to meet the ERC721
@@ -11,6 +12,7 @@ import './MixinLockCore.sol';
  * separates logically groupings of code to ease readability.
  */
 contract MixinKeys is
+  MixinErrors,
   MixinLockCore
 {
   // The struct for a key
@@ -96,12 +98,13 @@ contract MixinKeys is
   internal
   view
   {
-    require(
-      _isKeyManager(_tokenId, msg.sender) ||
-      approved[_tokenId] == msg.sender ||
-      isApprovedForAll(_ownerOf[_tokenId], msg.sender),
-      'ONLY_KEY_MANAGER_OR_APPROVED'
-    );
+    if(
+      !_isKeyManager(_tokenId, msg.sender)
+      && approved[_tokenId] != msg.sender
+      && !isApprovedForAll(_ownerOf[_tokenId], msg.sender)
+    ) {
+      revert ONLY_KEY_MANAGER_OR_APPROVED();
+    }
   }
 
   /**
@@ -114,10 +117,10 @@ contract MixinKeys is
   internal
   view
   {
-    require(
-      isValidKey(_tokenId),
-      'KEY_NOT_VALID'
-    );
+    if(!isValidKey(_tokenId)) {
+      revert KEY_NOT_VALID();
+    }
+    
   }
 
   /**
@@ -130,9 +133,9 @@ contract MixinKeys is
   internal
   view 
   {
-    require(
-      _keys[_tokenId].expirationTimestamp != 0, 'NO_SUCH_KEY'
-    );
+    if(_keys[_tokenId].expirationTimestamp == 0) {
+      revert NO_SUCH_KEY();
+    }
   }
 
   /**
@@ -187,7 +190,9 @@ contract MixinKeys is
     view
     returns (uint256)
   {
-      require(_index < balanceOf(_keyOwner), "OWNER_INDEX_OUT_OF_BOUNDS");
+      if(_index >= balanceOf(_keyOwner)) {
+        revert OUT_OF_RANGE();
+      }
       return _ownedKeyIds[_keyOwner][_index];
   }
 
@@ -239,7 +244,9 @@ contract MixinKeys is
     uint expirationTimestamp = _keys[_tokenId].expirationTimestamp;
 
     // prevent extending a valid non-expiring key
-    require(expirationTimestamp != type(uint).max, 'CANT_EXTEND_NON_EXPIRING_KEY');
+    if(expirationTimestamp == type(uint).max){
+      revert CANT_EXTEND_NON_EXPIRING_KEY();
+    }
     
     // if non-expiring but not valid then extend
     if(expirationDuration == type(uint).max) {
@@ -271,7 +278,9 @@ contract MixinKeys is
     uint length = balanceOf(_recipient);
     
     // make sure address does not have more keys than allowed
-    require(length < _maxKeysPerAddress, 'MAX_KEYS');
+    if(length >= _maxKeysPerAddress) {
+      revert MAX_KEYS_REACHED();
+    }
 
     // record new owner
     _ownedKeysIndex[_tokenId] = length;
@@ -301,7 +310,11 @@ contract MixinKeys is
     _isKey(_tokenIdTo);
     
     // make sure there is enough time remaining
-    require(keyExpirationTimestampFor(_tokenIdFrom) - block.timestamp >= _amount, 'NOT_ENOUGH_TIME');
+    if(
+      _amount > keyExpirationTimestampFor(_tokenIdFrom) - block.timestamp
+    ) {
+      revert NOT_ENOUGH_TIME();
+    }
 
     // deduct time from parent key
     _timeMachine(_tokenIdFrom, _amount, false);
@@ -373,7 +386,9 @@ contract MixinKeys is
     view
     returns (uint)
   {
-    require(_keyOwner != address(0), 'INVALID_ADDRESS');
+    if(_keyOwner == address(0)) { 
+      revert INVALID_ADDRESS();
+    }
     return _balances[_keyOwner];
   }
 
@@ -463,11 +478,12 @@ contract MixinKeys is
   ) public
   {
     _isKey(_tokenId);
-    require(
-      _isKeyManager(_tokenId, msg.sender) ||
-      isLockManager(msg.sender),
-      'UNAUTHORIZED_KEY_MANAGER_UPDATE'
-    );
+    if(
+      !_isKeyManager(_tokenId, msg.sender) 
+      && !isLockManager(msg.sender)
+    ) {
+      revert UNAUTHORIZED_KEY_MANAGER_UPDATE();
+    }
     _setKeyManagerOf(_tokenId, _keyManager);
   }
 
@@ -495,7 +511,9 @@ contract MixinKeys is
     public
   {
     _onlyKeyManagerOrApproved(_tokenId);
-    require(msg.sender != _approved, 'APPROVE_SELF');
+    if(msg.sender == _approved) {
+      revert CANNOT_APPROVE_SELF();
+    }
 
     approved[_tokenId] = _approved;
     emit Approval(_ownerOf[_tokenId], _approved, _tokenId);
@@ -605,7 +623,9 @@ contract MixinKeys is
    */
   function setMaxNumberOfKeys (uint _maxNumberOfKeys) external {
      _onlyLockManager();
-     require (_maxNumberOfKeys >= _totalSupply, "SMALLER_THAN_SUPPLY");
+     if (_maxNumberOfKeys < _totalSupply) {
+       revert CANT_BE_SMALLER_THAN_SUPPLY();
+     }
      maxNumberOfKeys = _maxNumberOfKeys;
   }
 
@@ -627,7 +647,9 @@ contract MixinKeys is
    */
   function setMaxKeysPerAddress(uint _maxKeys) external {
      _onlyLockManager();
-     require(_maxKeys != 0, 'NULL_VALUE');
+     if(_maxKeys == 0) {
+       revert NULL_VALUE();
+     }
      _maxKeysPerAddress = _maxKeys;
   }
 
diff --git a/smart-contracts/contracts/mixins/MixinLockCore.sol b/smart-contracts/contracts/mixins/MixinLockCore.sol
index 0eabde2c9c1..16b7364eefe 100644
--- a/smart-contracts/contracts/mixins/MixinLockCore.sol
+++ b/smart-contracts/contracts/mixins/MixinLockCore.sol
@@ -5,6 +5,7 @@ pragma solidity ^0.8.0;
 import '@openzeppelin/contracts-upgradeable/utils/AddressUpgradeable.sol';
 import './MixinDisable.sol';
 import './MixinRoles.sol';
+import './MixinErrors.sol';
 import '../interfaces/IUnlock.sol';
 import './MixinFunds.sol';
 import '../interfaces/hooks/ILockKeyCancelHook.sol';
@@ -91,10 +92,9 @@ contract MixinLockCore is
 
   // modifier to check if data has been upgraded
   function _lockIsUpToDate() internal view {
-    require(
-      schemaVersion == publicLockVersion(),
-      'MIGRATION_REQUIRED'
-    );
+    if(schemaVersion != publicLockVersion()) {
+      revert MIGRATION_REQUIRED();
+    }
   }
 
   // modifier
@@ -102,10 +102,9 @@ contract MixinLockCore is
   internal 
   view
   {
-    require(
-      isLockManager(msg.sender) || msg.sender == beneficiary,
-      'ONLY_LOCK_MANAGER_OR_BENEFICIARY'
-    );
+    if(!isLockManager(msg.sender) && msg.sender != beneficiary) {
+      revert ONLY_LOCK_MANAGER_OR_BENEFICIARY();
+    }
   }
   
   function _initializeMixinLockCore(
@@ -161,7 +160,9 @@ contract MixinLockCore is
     uint amount;
     if(_amount == 0 || _amount > balance)
     {
-      require(balance > 0, 'NOT_ENOUGH_FUNDS');
+      if(balance <= 0) {
+        revert NOT_ENOUGH_FUNDS();
+      }
       amount = balance;
     }
     else
@@ -202,7 +203,9 @@ contract MixinLockCore is
     address payable _beneficiary
   ) external {
     _onlyLockManagerOrBeneficiary();
-    require(_beneficiary != address(0), 'INVALID_ADDRESS');
+    if(_beneficiary == address(0)) {
+      revert INVALID_ADDRESS();
+    }
     beneficiary = _beneficiary;
   }
 
@@ -217,10 +220,12 @@ contract MixinLockCore is
   ) external
   {
     _onlyLockManager();
-    require(_onKeyPurchaseHook == address(0) || _onKeyPurchaseHook.isContract(), 'INVALID_ON_KEY_SOLD_HOOK');
-    require(_onKeyCancelHook == address(0) || _onKeyCancelHook.isContract(), 'INVALID_ON_KEY_CANCEL_HOOK');
-    require(_onValidKeyHook == address(0) || _onValidKeyHook.isContract(), 'INVALID_ON_VALID_KEY_HOOK');
-    require(_onTokenURIHook == address(0) || _onTokenURIHook.isContract(), 'INVALID_ON_TOKEN_URI_HOOK');
+
+    if(_onKeyPurchaseHook != address(0) && !_onKeyPurchaseHook.isContract()) { revert INVALID_HOOK(0); }
+    if(_onKeyCancelHook != address(0) && !_onKeyCancelHook.isContract()) { revert INVALID_HOOK(1); }
+    if(_onValidKeyHook != address(0) && !_onValidKeyHook.isContract()) { revert INVALID_HOOK(2); }
+    if(_onTokenURIHook != address(0) && !_onTokenURIHook.isContract()) { revert INVALID_HOOK(3); }
+    
     onKeyPurchaseHook = ILockKeyPurchaseHook(_onKeyPurchaseHook);
     onKeyCancelHook = ILockKeyCancelHook(_onKeyCancelHook);
     onTokenURIHook = ILockTokenURIHook(_onTokenURIHook);
diff --git a/smart-contracts/contracts/mixins/MixinPurchase.sol b/smart-contracts/contracts/mixins/MixinPurchase.sol
index c611ab50d9b..c774b0c6790 100644
--- a/smart-contracts/contracts/mixins/MixinPurchase.sol
+++ b/smart-contracts/contracts/mixins/MixinPurchase.sol
@@ -5,6 +5,7 @@ import './MixinDisable.sol';
 import './MixinKeys.sol';
 import './MixinLockCore.sol';
 import './MixinFunds.sol';
+import './MixinErrors.sol';
 
 /**
  * @title Mixin for the purchase-related functions.
@@ -13,6 +14,7 @@ import './MixinFunds.sol';
  * separates logically groupings of code to ease readability.
  */
 contract MixinPurchase is
+  MixinErrors,
   MixinFunds,
   MixinDisable,
   MixinLockCore,
@@ -92,9 +94,16 @@ contract MixinPurchase is
   ) external payable
   {
     _lockIsUpToDate();
-    require(maxNumberOfKeys > _totalSupply, 'LOCK_SOLD_OUT');
-    require(_recipients.length == _referrers.length, 'INVALID_REFERRERS_LENGTH');
-    require(_recipients.length == _keyManagers.length, 'INVALID_KEY_MANAGERS_LENGTH');
+    if(_totalSupply >= maxNumberOfKeys) {
+      revert LOCK_SOLD_OUT();
+    }
+    if(
+      (_recipients.length != _referrers.length)
+      ||
+      (_recipients.length != _keyManagers.length)
+      ) {
+      revert INVALID_LENGTH();
+    }
 
     uint totalPriceToPay;
     uint tokenId;
@@ -102,7 +111,9 @@ contract MixinPurchase is
     for (uint256 i = 0; i < _recipients.length; i++) {
       // check recipient address
       address _recipient = _recipients[i];
-      require(_recipient != address(0), 'INVALID_ADDRESS');
+      if(_recipient == address(0)) {
+        revert INVALID_ADDRESS();
+      }
       
       // check for a non-expiring key
       if (expirationDuration == type(uint).max) {
@@ -120,8 +131,7 @@ contract MixinPurchase is
         );
       }
 
-      // price      
-
+      // price
       uint inMemoryKeyPrice = purchasePriceFor(_recipient, _referrers[i], _data[i]);
       totalPriceToPay = totalPriceToPay + inMemoryKeyPrice;
 
@@ -130,8 +140,8 @@ contract MixinPurchase is
       _originalDurations[tokenId] = expirationDuration;
       _originalTokens[tokenId] = tokenAddress;
       
-      if(tokenAddress != address(0)) {
-        require(inMemoryKeyPrice <= _values[i], 'INSUFFICIENT_ERC20_VALUE');
+      if(tokenAddress != address(0) && _values[i] < inMemoryKeyPrice) {
+        revert INSUFFICIENT_ERC20_VALUE();
       }
 
       // store in unlock
@@ -150,14 +160,13 @@ contract MixinPurchase is
         );
       }
     }
-
     // transfer the ERC20 tokens
     if(tokenAddress != address(0)) {
       IERC20Upgradeable token = IERC20Upgradeable(tokenAddress);
       token.transferFrom(msg.sender, address(this), totalPriceToPay);
-    } else {
+    } else if(msg.value < totalPriceToPay) {
       // We explicitly allow for greater amounts of ETH or tokens to allow 'donations'
-      require(totalPriceToPay <= msg.value, 'INSUFFICIENT_VALUE');
+      revert INSUFFICIENT_VALUE();
     }
 
     // refund gas
@@ -192,12 +201,14 @@ contract MixinPurchase is
     uint inMemoryKeyPrice = purchasePriceFor(ownerOf(_tokenId), _referrer, _data);
 
     if(tokenAddress != address(0)) {
-      require(inMemoryKeyPrice <= _value, 'INSUFFICIENT_ERC20_VALUE');
+      if(_value < inMemoryKeyPrice) {
+        revert INSUFFICIENT_ERC20_VALUE();
+      }
       IERC20Upgradeable token = IERC20Upgradeable(tokenAddress);
       token.transferFrom(msg.sender, address(this), inMemoryKeyPrice);
-    } else {
+    } else if(msg.value < inMemoryKeyPrice) {
       // We explicitly allow for greater amounts of ETH or tokens to allow 'donations'
-      require(inMemoryKeyPrice <= msg.value, 'INSUFFICIENT_VALUE');
+      revert INSUFFICIENT_VALUE();
     }
 
     // refund gas (if applicable)
@@ -218,17 +229,26 @@ contract MixinPurchase is
     _isKey(_tokenId);
 
     // check the lock
-    require(_originalDurations[_tokenId] != type(uint).max, 'NON_EXPIRING_LOCK');
-    require(tokenAddress != address(0), 'NON_ERC20_LOCK');
+    if(_originalDurations[_tokenId] == type(uint).max || tokenAddress == address(0)) {
+      revert NON_RENEWABLE_LOCK();
+    }
 
     // make sure duration and pricing havent changed  
     uint keyPrice = purchasePriceFor(ownerOf(_tokenId), _referrer, '');
-    require(_originalPrices[_tokenId] == keyPrice, 'PRICE_CHANGED');
-    require(_originalDurations[_tokenId] == expirationDuration, 'DURATION_CHANGED');
-    require(_originalTokens[_tokenId] == tokenAddress, 'TOKEN_CHANGED');
+    if(
+      _originalPrices[_tokenId] != keyPrice
+      ||
+      _originalDurations[_tokenId] != expirationDuration
+      || 
+      _originalTokens[_tokenId] != tokenAddress
+    ) {
+      revert LOCK_HAS_CHANGED();
+    }
 
     // make sure key is ready for renewal
-    require(isValidKey(_tokenId) == false, 'NOT_READY');
+    if(isValidKey(_tokenId)) {
+      revert NOT_READY_FOR_RENEWAL();
+    }
 
     // extend key duration
     _extendKey(_tokenId);
@@ -276,7 +296,9 @@ contract MixinPurchase is
         token.transferFrom(address(this), msg.sender, _gasRefundValue);
       } else {
         (bool success, ) = msg.sender.call{value: _gasRefundValue}("");
-        require(success, "REFUND_FAILED");
+        if(!success) {
+          revert GAS_REFUND_FAILED();
+        }
       }
       emit GasRefunded(msg.sender, _gasRefundValue, tokenAddress);
     }
diff --git a/smart-contracts/contracts/mixins/MixinRoles.sol b/smart-contracts/contracts/mixins/MixinRoles.sol
index 6648899a998..127e33a58aa 100644
--- a/smart-contracts/contracts/mixins/MixinRoles.sol
+++ b/smart-contracts/contracts/mixins/MixinRoles.sol
@@ -5,8 +5,9 @@ pragma solidity ^0.8.0;
 // openzeppelin/contracts-ethereum-package/contracts/access/roles
 
 import '@openzeppelin/contracts-upgradeable/access/AccessControlUpgradeable.sol';
+import './MixinErrors.sol';
 
-contract MixinRoles is AccessControlUpgradeable {
+contract MixinRoles is AccessControlUpgradeable, MixinErrors {
 
   // roles
   bytes32 public constant LOCK_MANAGER_ROLE = keccak256("LOCK_MANAGER");
@@ -41,7 +42,9 @@ contract MixinRoles is AccessControlUpgradeable {
   internal 
   view
   {
-    require( hasRole(LOCK_MANAGER_ROLE, msg.sender), 'ONLY_LOCK_MANAGER');
+    if(!hasRole(LOCK_MANAGER_ROLE, msg.sender)) {
+      revert ONLY_LOCK_MANAGER();
+    }
   }
 
   // lock manager functions
diff --git a/smart-contracts/contracts/mixins/MixinTransfer.sol b/smart-contracts/contracts/mixins/MixinTransfer.sol
index 63b570875f1..69f44752d2d 100644
--- a/smart-contracts/contracts/mixins/MixinTransfer.sol
+++ b/smart-contracts/contracts/mixins/MixinTransfer.sol
@@ -7,6 +7,7 @@ import './MixinKeys.sol';
 import './MixinFunds.sol';
 import './MixinLockCore.sol';
 import './MixinPurchase.sol';
+import './MixinErrors.sol';
 import '@openzeppelin/contracts-upgradeable/token/ERC721/IERC721ReceiverUpgradeable.sol';
 import '@openzeppelin/contracts-upgradeable/utils/AddressUpgradeable.sol';
 
@@ -19,6 +20,7 @@ import '@openzeppelin/contracts-upgradeable/utils/AddressUpgradeable.sol';
  */
 
 contract MixinTransfer is
+  MixinErrors,
   MixinRoles,
   MixinFunds,
   MixinLockCore,
@@ -53,13 +55,21 @@ contract MixinTransfer is
   ) public
   {
     _lockIsUpToDate();
-    require(maxNumberOfKeys > _totalSupply, 'LOCK_SOLD_OUT');
+    if(maxNumberOfKeys <= _totalSupply) {
+      revert LOCK_SOLD_OUT();
+    }
     _onlyKeyManagerOrApproved(_tokenIdFrom);
     _isValidKey(_tokenIdFrom);
-    require(transferFeeBasisPoints < BASIS_POINTS_DEN, 'KEY_TRANSFERS_DISABLED');
-    require(_to != address(0), 'INVALID_ADDRESS');
+    if(transferFeeBasisPoints >= BASIS_POINTS_DEN) {
+      revert KEY_TRANSFERS_DISABLED();
+    }
+    if(_to == address(0)) {
+      revert INVALID_ADDRESS();
+    } 
     address keyOwner = _ownerOf[_tokenIdFrom];
-    require(keyOwner != _to, 'TRANSFER_TO_SELF');
+    if(keyOwner == _to) {
+      revert TRANSFER_TO_SELF();
+    }
 
     // store time to be added
     uint time;
@@ -99,7 +109,9 @@ contract MixinTransfer is
       tokenIdTo
     );
 
-    require(_checkOnERC721Received(keyOwner, _to, tokenIdTo, ''), 'NON_COMPLIANT_ERC721_RECEIVER');
+    if(!_checkOnERC721Received(keyOwner, _to, tokenIdTo, '')) {
+      revert NON_COMPLIANT_ERC721_RECEIVER();
+    }
   }
 
 
@@ -112,10 +124,18 @@ contract MixinTransfer is
   {
     _isValidKey(_tokenId);
     _onlyKeyManagerOrApproved(_tokenId);
-    require(ownerOf(_tokenId) == _from, 'TRANSFER_FROM: NOT_KEY_OWNER');
-    require(transferFeeBasisPoints < BASIS_POINTS_DEN, 'KEY_TRANSFERS_DISABLED');
-    require(_recipient != address(0), 'INVALID_ADDRESS');
-    require(_from != _recipient, 'TRANSFER_TO_SELF');
+    if(ownerOf(_tokenId) != _from) {
+      revert UNAUTHORIZED();
+    }
+    if(transferFeeBasisPoints >= BASIS_POINTS_DEN) {
+      revert KEY_TRANSFERS_DISABLED();
+    }
+    if(_recipient == address(0)) {
+      revert INVALID_ADDRESS();
+    }
+    if(_from == _recipient) {
+      revert TRANSFER_TO_SELF();
+    }
 
     // subtract the fee from the senders key before the transfer
     _timeMachine(_tokenId, getTransferFee(_tokenId, 0), false);  
@@ -203,8 +223,12 @@ contract MixinTransfer is
     bool _approved
   ) public
   {
-    require(_to != msg.sender, 'APPROVE_SELF');
-    require(transferFeeBasisPoints < BASIS_POINTS_DEN, 'KEY_TRANSFERS_DISABLED');
+    if(_to == msg.sender) {
+      revert CANNOT_APPROVE_SELF();
+    }
+    if(transferFeeBasisPoints >= BASIS_POINTS_DEN) {
+      revert KEY_TRANSFERS_DISABLED();
+    }
     managerToOperatorApproved[msg.sender][_to] = _approved;
     emit ApprovalForAll(msg.sender, _to, _approved);
   }
@@ -229,8 +253,9 @@ contract MixinTransfer is
     public
   {
     transferFrom(_from, _to, _tokenId);
-    require(_checkOnERC721Received(_from, _to, _tokenId, _data), 'NON_COMPLIANT_ERC721_RECEIVER');
-
+    if(!_checkOnERC721Received(_from, _to, _tokenId, _data)) {
+      revert NON_COMPLIANT_ERC721_RECEIVER();
+    }
   }
 
   /**
diff --git a/smart-contracts/test/Lock/erc721/transferFrom.js b/smart-contracts/test/Lock/erc721/transferFrom.js
index ba253e8069e..3bf9d62ab47 100644
--- a/smart-contracts/test/Lock/erc721/transferFrom.js
+++ b/smart-contracts/test/Lock/erc721/transferFrom.js
@@ -74,7 +74,7 @@ contract('Lock / erc721 / transferFrom', (accounts) => {
         locks.FIRST.transferFrom(keyOwners[2], accounts[9], tokenIds[0], {
           from: keyOwners[0],
         }),
-        'TRANSFER_FROM: NOT_KEY_OWNER'
+        'UNAUTHORIZED'
       )
     })
 
diff --git a/smart-contracts/test/Lock/onKeyCancelHook.js b/smart-contracts/test/Lock/onKeyCancelHook.js
index a8cd3fc4c19..a16340c418f 100644
--- a/smart-contracts/test/Lock/onKeyCancelHook.js
+++ b/smart-contracts/test/Lock/onKeyCancelHook.js
@@ -56,7 +56,7 @@ contract('Lock / onKeyCancelHook', (accounts) => {
   it('cannot set the hook to a non-contract address', async () => {
     await reverts(
       lock.setEventHooks(ADDRESS_ZERO, accounts[1], ADDRESS_ZERO, ADDRESS_ZERO),
-      'INVALID_ON_KEY_CANCEL_HOOK'
+      'INVALID_HOOK(1)'
     )
   })
 })
diff --git a/smart-contracts/test/Lock/onKeyPurchaseHook.js b/smart-contracts/test/Lock/onKeyPurchaseHook.js
index e7547972f52..9915da7f9c1 100644
--- a/smart-contracts/test/Lock/onKeyPurchaseHook.js
+++ b/smart-contracts/test/Lock/onKeyPurchaseHook.js
@@ -88,7 +88,7 @@ contract('Lock / onKeyPurchaseHook', (accounts) => {
           ADDRESS_ZERO,
           ADDRESS_ZERO
         ),
-        'INVALID_ON_KEY_SOLD_HOOK'
+        'INVALID_HOOK(0)'
       )
     })
   })
diff --git a/smart-contracts/test/Lock/onTokenURIHook.js b/smart-contracts/test/Lock/onTokenURIHook.js
index 934f80a1d5a..10dd8cc8777 100644
--- a/smart-contracts/test/Lock/onTokenURIHook.js
+++ b/smart-contracts/test/Lock/onTokenURIHook.js
@@ -61,7 +61,7 @@ contract('Lock / onTokenURIHook', (accounts) => {
   it('cannot set the hook to a non-contract address', async () => {
     await reverts(
       lock.setEventHooks(ADDRESS_ZERO, ADDRESS_ZERO, ADDRESS_ZERO, accounts[3]),
-      'INVALID_ON_TOKEN_URI_HOOK'
+      'INVALID_HOOK(3)'
     )
   })
 })
diff --git a/smart-contracts/test/Lock/onValidKeyHook.js b/smart-contracts/test/Lock/onValidKeyHook.js
index 3581f4efb91..53a190290d9 100644
--- a/smart-contracts/test/Lock/onValidKeyHook.js
+++ b/smart-contracts/test/Lock/onValidKeyHook.js
@@ -62,7 +62,7 @@ contract('Lock / onValidKeyHook', (accounts) => {
   it('cannot set the hook to a non-contract address', async () => {
     await reverts(
       lock.setEventHooks(ADDRESS_ZERO, ADDRESS_ZERO, accounts[3], ADDRESS_ZERO),
-      'INVALID_ON_VALID_KEY_HOOK'
+      'INVALID_HOOK(2)'
     )
   })
 })
diff --git a/smart-contracts/test/Lock/renewMembershipFor.js b/smart-contracts/test/Lock/renewMembershipFor.js
index d5fe5246936..4bb7e3e4f78 100644
--- a/smart-contracts/test/Lock/renewMembershipFor.js
+++ b/smart-contracts/test/Lock/renewMembershipFor.js
@@ -81,7 +81,7 @@ contract('Lock / Recurring memberships', (accounts) => {
 
         await reverts(
           locks.NON_EXPIRING.renewMembershipFor(newTokenId, ADDRESS_ZERO),
-          'NON_EXPIRING_LOCK'
+          'NON_RENEWABLE_LOCK'
         )
       })
 
@@ -103,7 +103,7 @@ contract('Lock / Recurring memberships', (accounts) => {
 
         await reverts(
           locks.FIRST.renewMembershipFor(newTokenId, ADDRESS_ZERO),
-          'NON_ERC20_LOCK'
+          'NON_RENEWABLE_LOCK'
         )
       })
     })
@@ -169,7 +169,7 @@ contract('Lock / Recurring memberships', (accounts) => {
         )
         await reverts(
           lock.renewMembershipFor(tokenId, ADDRESS_ZERO),
-          'PRICE_CHANGED'
+          'LOCK_HAS_CHANGED'
         )
       })
 
@@ -183,7 +183,7 @@ contract('Lock / Recurring memberships', (accounts) => {
         await lock.updateKeyPricing(keyPrice, dai2.address, { from: lockOwner })
         await reverts(
           lock.renewMembershipFor(tokenId, ADDRESS_ZERO),
-          'TOKEN_CHANGED'
+          'LOCK_HAS_CHANGED'
         )
       })
 
@@ -191,7 +191,7 @@ contract('Lock / Recurring memberships', (accounts) => {
         await lock.setExpirationDuration(1000, { from: lockOwner })
         await reverts(
           lock.renewMembershipFor(tokenId, ADDRESS_ZERO),
-          'DURATION_CHANGED'
+          'LOCK_HAS_CHANGED'
         )
       })
     })
@@ -285,7 +285,7 @@ contract('Lock / Recurring memberships', (accounts) => {
         // should fail
         await reverts(
           lock.renewMembershipFor(tokenId, ADDRESS_ZERO),
-          'DURATION_CHANGED'
+          'LOCK_HAS_CHANGED'
         )
       })
 
@@ -322,7 +322,7 @@ contract('Lock / Recurring memberships', (accounts) => {
 
         await reverts(
           lock.renewMembershipFor(tokenId, ADDRESS_ZERO),
-          'DURATION_CHANGED'
+          'LOCK_HAS_CHANGED'
         )
       })
     })
diff --git a/smart-contracts/test/Lock/timeMachine.js b/smart-contracts/test/Lock/timeMachine.js
index df45da60061..fc5fb246162 100644
--- a/smart-contracts/test/Lock/timeMachine.js
+++ b/smart-contracts/test/Lock/timeMachine.js
@@ -3,11 +3,9 @@ const BigNumber = require('bignumber.js')
 
 const TimeMachineMock = artifacts.require('TimeMachineMock')
 
-const { errorMessages, ADDRESS_ZERO } = require('../helpers/constants')
+const { ADDRESS_ZERO } = require('../helpers/constants')
 const { reverts } = require('../helpers/errors')
 
-const { VM_ERROR_REVERT_WITH_REASON } = errorMessages
-
 contract('Lock / timeMachine', (accounts) => {
   let timeMachine
   const keyOwner = accounts[2]
@@ -92,7 +90,7 @@ contract('Lock / timeMachine', (accounts) => {
     it('should not work for a non-existant key', async () => {
       await reverts(
         timeMachine.timeMachine(17, 42, true, { from: accounts[3] }),
-        `${VM_ERROR_REVERT_WITH_REASON} 'NO_SUCH_KEY'`
+        'NO_SUCH_KEY'
       )
     })
   })
diff --git a/smart-contracts/test/Lock/transfer.js b/smart-contracts/test/Lock/transfer.js
index 552676a78ef..9044cc08065 100644
--- a/smart-contracts/test/Lock/transfer.js
+++ b/smart-contracts/test/Lock/transfer.js
@@ -4,9 +4,7 @@ const deployLocks = require('../helpers/deployLocks')
 
 const unlockContract = artifacts.require('Unlock.sol')
 const getProxy = require('../helpers/proxy')
-const { errorMessages, ADDRESS_ZERO } = require('../helpers/constants')
-
-const { VM_ERROR_REVERT_WITH_REASON } = errorMessages
+const { ADDRESS_ZERO } = require('../helpers/constants')
 
 let unlock
 let lock
@@ -115,7 +113,7 @@ contract('Lock / transfer', (accounts) => {
       lock.transfer(tokenIds[0], singleKeyOwner, 1000, {
         from: singleKeyOwner,
       }),
-      `${VM_ERROR_REVERT_WITH_REASON} 'TRANSFER_TO_SELF'`
+      'TRANSFER_TO_SELF'
     )
   })