diff --git a/l1-contracts/src/core/Decoder.sol b/l1-contracts/src/core/Decoder.sol
index b1ee91420e8..9ba6ae6ab92 100644
--- a/l1-contracts/src/core/Decoder.sol
+++ b/l1-contracts/src/core/Decoder.sol
@@ -48,12 +48,15 @@ pragma solidity >=0.8.18;
  *  | 0x1bc + x + y            | 0x04       | len(newContracts) denoted z
  *  | 0x1c0 + x + y            | z          | newContracts
  *  | 0x1c0 + x + y + z        | z          | newContractData
+ *  | 0x1c0 + x + y + z        | 0x04       | len(l1ToL2Messages) denoted w
+ *  | 0x1c4 + x + y + z        | w          | l1ToL2Messages
  *  |---                       |---         | ---
  *
  */
 contract Decoder {
   uint256 internal constant COMMITMENTS_PER_KERNEL = 4;
   uint256 internal constant NULLIFIERS_PER_KERNEL = 4;
+  uint256 internal constant L1_TO_L2_MESSAGES_PER_ROLLUP = 16;
 
   /**
    * @notice Decodes the inputs and computes values to check state against
@@ -63,6 +66,7 @@ contract Decoder {
    * @return newStateHash - The state hash expected after the execution.
    * @return publicInputHash - The hash of the public inputs
    */
+  // TODO: temporarily to get this going, the l1ToL2Messages are included here
   function _decode(bytes calldata _l2Block)
     internal
     pure
@@ -89,18 +93,23 @@ contract Decoder {
   function _computePublicInputsHash(bytes calldata _l2Block) internal pure returns (bytes32) {
     // Compute the public inputs hash
     uint256 size = 0x194 + 0x20;
+    // block header
     bytes memory temp = new bytes(size);
     assembly {
       calldatacopy(add(temp, 0x20), add(_l2Block.offset, 0x20), size)
     }
 
-    bytes32 diffRoot = _computeDiffRoot(_l2Block);
+    // Diff root
+    (bytes32 diffRoot, bytes32 l1ToL2messagesHash) = _computeDiffRoot(_l2Block);
     assembly {
       mstore(add(temp, add(0x20, 0x194)), diffRoot)
+      mstore(add(temp, add(0x20, 0x1a4)), l1ToL2messagesHash)
     }
+
     return sha256(temp);
   }
 
+
   /**
    * @notice Extract the L2 block number from the block
    * @param _l2Block - The L2 block calldata
@@ -150,7 +159,7 @@ contract Decoder {
    * @param _l2Block - The L2 block calldata.
    * @return The root of the "diff" tree
    */
-  function _computeDiffRoot(bytes calldata _l2Block) internal pure returns (bytes32) {
+  function _computeDiffRoot(bytes calldata _l2Block) internal pure returns (bytes32 _diffRoot, bytes32 _l1ToL2MessagesHash) {
     Vars memory vars;
     {
       uint256 commitmentCount;
@@ -183,6 +192,8 @@ contract Decoder {
       0x178 + (baseLeafs.length * 2 * (NULLIFIERS_PER_KERNEL + COMMITMENTS_PER_KERNEL) * 0x20);
     uint256 srcContractDataOffset = srcContractOffset + vars.contractCount * 0x20;
 
+    uint256 srcL1ToL2MessagesDataOffset = srcContractDataOffset + vars.contractCount * 0x40;
+
     for (uint256 i = 0; i < baseLeafs.length; i++) {
       /**
        * Compute the leaf to insert.
@@ -255,7 +266,17 @@ contract Decoder {
       baseLeafs[i] = sha256(baseLeaf);
     }
 
-    return _computeRoot(baseLeafs);
+    _diffRoot = _computeRoot(baseLeafs);
+
+    // TODO: tidy this up
+    // TODO: calcualte the offset for the messages, might be easier to do in the compute diff root method as it gets all of that stuff
+    uint256 size = 0x20 * L1_TO_L2_MESSAGES_PER_ROLLUP;
+    bytes memory temp = new bytes(size);
+    assembly {
+      calldatacopy(add(temp, 0x20), add(srcL1ToL2MessagesDataOffset, 0x20), size)
+    }
+    _l1ToL2MessagesHash =  sha256(temp);
+
   }
 
   /**
diff --git a/yarn-project/types/src/l2_block.ts b/yarn-project/types/src/l2_block.ts
index 7492100984a..df9142a769f 100644
--- a/yarn-project/types/src/l2_block.ts
+++ b/yarn-project/types/src/l2_block.ts
@@ -3,6 +3,7 @@ import {
   KERNEL_NEW_COMMITMENTS_LENGTH,
   KERNEL_NEW_CONTRACTS_LENGTH,
   KERNEL_NEW_NULLIFIERS_LENGTH,
+  NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP,
 } from '@aztec/circuits.js';
 import { makeAppendOnlyTreeSnapshot } from '@aztec/circuits.js/factories';
 import { BufferReader, serializeToBuffer } from '@aztec/circuits.js/utils';
@@ -58,6 +59,7 @@ export class L2Block {
     public newNullifiers: Fr[],
     public newContracts: Fr[],
     public newContractData: ContractData[],
+    public newL1ToL2Messages: Fr[] = [],
   ) {}
 
   // TODO: do the newMessages need to be public inputs for stuff
@@ -74,6 +76,9 @@ export class L2Block {
     const newContractsData = Array(KERNEL_NEW_CONTRACTS_LENGTH * txsPerBlock)
       .fill(0)
       .map(() => ContractData.random());
+    const newL1ToL2Messages = Array(NUMBER_OF_L1_L2_MESSAGES_PER_ROLLUP)
+      .fill(0)
+      .map(() => Fr.random());
 
     return new L2Block(
       l2BlockNum,
@@ -95,6 +100,7 @@ export class L2Block {
       newNullifiers,
       newContracts,
       newContractsData,
+      newL1ToL2Messages,
     );
   }
 
@@ -123,6 +129,7 @@ export class L2Block {
     newNullifiers: Fr[];
     newContracts: Fr[];
     newContractData: ContractData[];
+    newL1ToL2Messages: Fr[];
   }) {
     return new this(
       fields.number,
@@ -144,6 +151,7 @@ export class L2Block {
       fields.newNullifiers,
       fields.newContracts,
       fields.newContractData,
+      fields.newL1ToL2Messages,
     );
   }
 
@@ -175,6 +183,7 @@ export class L2Block {
       this.newContracts.length,
       this.newContracts,
       this.newContractData,
+      this.newL1ToL2Messages,
     );
   }
 
@@ -212,6 +221,8 @@ export class L2Block {
     const newNullifiers = reader.readVector(Fr);
     const newContracts = reader.readVector(Fr);
     const newContractData = reader.readArray(newContracts.length, ContractData);
+    // could an optimisation of this be that it is encoded such that zeros are assumed
+    const newL1ToL2Messages = reader.readVector(Fr);
 
     return new L2Block(
       number,
@@ -233,6 +244,7 @@ export class L2Block {
       newNullifiers,
       newContracts,
       newContractData,
+      newL1ToL2Messages,
     );
   }
 
@@ -321,6 +333,7 @@ export class L2Block {
       `newNullifiers: ${inspectFrArray(this.newNullifiers)}`,
       `newContracts: ${inspectFrArray(this.newContracts)}`,
       `newContractData: ${inspectContractDataArray(this.newContractData)}`,
+      `newL1ToL2Messages: ${inspectFrArray(this.newL1ToL2Messages)}`,
     ].join('\n');
   }
 }