diff --git a/crates/common/src/commit/response.rs b/crates/common/src/commit/response.rs
index e1096828..5424a4bd 100644
--- a/crates/common/src/commit/response.rs
+++ b/crates/common/src/commit/response.rs
@@ -2,7 +2,7 @@ use std::{fmt, fmt::Display};
 
 use alloy::{
     hex,
-    primitives::{Address, B256},
+    primitives::{Address, B256, U256},
     rpc::types::beacon::BlsSignature,
 };
 use serde::{Deserialize, Serialize};
@@ -15,7 +15,7 @@ pub struct BlsSignResponse {
     pub object_root: B256,
     pub module_signing_id: B256,
     pub nonce: u64,
-    pub chain_id: u64,
+    pub chain_id: U256,
     pub signature: BlsSignature,
 }
 
@@ -25,7 +25,7 @@ impl BlsSignResponse {
         object_root: B256,
         module_signing_id: B256,
         nonce: u64,
-        chain_id: u64,
+        chain_id: U256,
         signature: BlsSignature,
     ) -> Self {
         Self { pubkey, object_root, module_signing_id, nonce, chain_id, signature }
@@ -53,7 +53,7 @@ pub struct EcdsaSignResponse {
     pub object_root: B256,
     pub module_signing_id: B256,
     pub nonce: u64,
-    pub chain_id: u64,
+    pub chain_id: U256,
     pub signature: EcdsaSignature,
 }
 
@@ -63,7 +63,7 @@ impl EcdsaSignResponse {
         object_root: B256,
         module_signing_id: B256,
         nonce: u64,
-        chain_id: u64,
+        chain_id: U256,
         signature: EcdsaSignature,
     ) -> Self {
         Self { address, object_root, module_signing_id, nonce, chain_id, signature }
diff --git a/crates/common/src/config/pbs.rs b/crates/common/src/config/pbs.rs
index d04b3394..e12493d4 100644
--- a/crates/common/src/config/pbs.rs
+++ b/crates/common/src/config/pbs.rs
@@ -169,12 +169,13 @@ impl PbsConfig {
             if !matches!(chain, Chain::Custom { .. }) {
                 let provider = ProviderBuilder::new().on_http(rpc_url.clone());
                 let chain_id = provider.get_chain_id().await?;
+                let chain_id_big = U256::from(chain_id);
                 ensure!(
-                    chain_id == chain.id(),
+                    chain_id_big == chain.id(),
                     "Rpc url is for the wrong chain, expected: {} ({:?}) got {}",
                     chain.id(),
                     chain,
-                    chain_id
+                    chain_id_big
                 );
             }
         }
diff --git a/crates/common/src/types.rs b/crates/common/src/types.rs
index 6a19be48..d650f6be 100644
--- a/crates/common/src/types.rs
+++ b/crates/common/src/types.rs
@@ -1,6 +1,6 @@
 use std::path::PathBuf;
 
-use alloy::primitives::{aliases::B32, hex, Bytes, B256};
+use alloy::primitives::{aliases::B32, hex, Bytes, B256, U256};
 use derive_more::{Deref, Display, From, Into};
 use eyre::{bail, Context};
 use serde::{Deserialize, Serialize};
@@ -72,7 +72,9 @@ impl std::fmt::Debug for Chain {
 }
 
 impl Chain {
-    pub fn id(&self) -> u64 {
+    // Chain IDs are 256-bit unsigned integers because they need to support
+    // Keccak256 hashes
+    pub fn id(&self) -> U256 {
         match self {
             Chain::Mainnet => KnownChain::Mainnet.id(),
             Chain::Holesky => KnownChain::Holesky.id(),
@@ -146,13 +148,13 @@ pub enum KnownChain {
 
 // Constants
 impl KnownChain {
-    pub fn id(&self) -> u64 {
+    pub fn id(&self) -> U256 {
         match self {
-            KnownChain::Mainnet => 1,
-            KnownChain::Holesky => 17000,
-            KnownChain::Sepolia => 11155111,
-            KnownChain::Helder => 167000,
-            KnownChain::Hoodi => 560048,
+            KnownChain::Mainnet => U256::from(1),
+            KnownChain::Holesky => U256::from(17000),
+            KnownChain::Sepolia => U256::from(11155111),
+            KnownChain::Helder => U256::from(167000),
+            KnownChain::Hoodi => U256::from(560048),
         }
     }
 
@@ -305,7 +307,7 @@ pub struct PropCommitSigningInfo {
     pub data: B256,
     pub module_signing_id: B256,
     pub nonce: u64, // As per https://eips.ethereum.org/EIPS/eip-2681
-    pub chain_id: u64,
+    pub chain_id: U256,
 }
 
 /// Information about a signature request, including the module signing ID and