diff --git a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcTestMethodsFactory.java b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcTestMethodsFactory.java
index 397978b519..3f23027ea6 100644
--- a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcTestMethodsFactory.java
+++ b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcTestMethodsFactory.java
@@ -49,7 +49,7 @@ public class JsonRpcTestMethodsFactory {
     this.importer = importer;
   }
 
-  public Map<String, JsonRpcMethod> methods(final String chainId) {
+  public Map<String, JsonRpcMethod> methods(final int chainId) {
     final WorldStateArchive stateArchive = InMemoryTestFixture.createInMemoryWorldStateArchive();
 
     importer.getGenesisConfig().writeStateTo(stateArchive.getMutable(Hash.EMPTY_TRIE_HASH));
diff --git a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthCallIntegrationTest.java b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthCallIntegrationTest.java
index d82da3bf20..04bd37583a 100644
--- a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthCallIntegrationTest.java
+++ b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthCallIntegrationTest.java
@@ -37,7 +37,7 @@ import org.junit.Test;
 
 public class EthCallIntegrationTest {
 
-  private static final String CHAIN_ID = "6986785976597";
+  private static final int CHAIN_ID = 123;
   private static JsonRpcTestMethodsFactory BLOCKCHAIN;
 
   private JsonRpcMethod method;
diff --git a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthEstimateGasIntegrationTest.java b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthEstimateGasIntegrationTest.java
index b3937e4755..427f9efbf2 100644
--- a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthEstimateGasIntegrationTest.java
+++ b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthEstimateGasIntegrationTest.java
@@ -33,7 +33,7 @@ import org.junit.Test;
 
 public class EthEstimateGasIntegrationTest {
 
-  private static final String CHAIN_ID = "6986785976597";
+  private static final int CHAIN_ID = 123;
   private static JsonRpcTestMethodsFactory BLOCKCHAIN;
 
   private JsonRpcMethod method;
diff --git a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetBlockByHashIntegrationTest.java b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetBlockByHashIntegrationTest.java
index d2eec237ec..7f39a1c063 100644
--- a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetBlockByHashIntegrationTest.java
+++ b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetBlockByHashIntegrationTest.java
@@ -58,7 +58,7 @@ public class EthGetBlockByHashIntegrationTest {
   private Map<String, JsonRpcMethod> methods;
   private static JsonRpcTestMethodsFactory BLOCKCHAIN;
   private final JsonRpcResponseUtils responseUtils = new JsonRpcResponseUtils();
-  private static final String CHAIN_ID = "6986785976597";
+  private static final int CHAIN_ID = 123;
   private final String ETH_METHOD = "eth_getBlockByHash";
   private final String JSON_RPC_VERSION = "2.0";
   private final String ZERO_HASH = String.valueOf(Hash.ZERO);
diff --git a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetBlockByNumberIntegrationTest.java b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetBlockByNumberIntegrationTest.java
index 34f54bd6ec..d662cf6e2d 100644
--- a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetBlockByNumberIntegrationTest.java
+++ b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetBlockByNumberIntegrationTest.java
@@ -59,7 +59,7 @@ import org.mockito.junit.MockitoJUnitRunner;
 @RunWith(MockitoJUnitRunner.class)
 public class EthGetBlockByNumberIntegrationTest {
 
-  private static final String CHAIN_ID = "6986785976597";
+  private static final int CHAIN_ID = 123;
   private static final String ETH_METHOD = "eth_getBlockByNumber";
   private static final String JSON_RPC_VERSION = "2.0";
   private static JsonRpcTestMethodsFactory BLOCKCHAIN;
diff --git a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetUncleByBlockHashAndIndexIntegrationTest.java b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetUncleByBlockHashAndIndexIntegrationTest.java
index 226619c9cd..63f77fe4d1 100644
--- a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetUncleByBlockHashAndIndexIntegrationTest.java
+++ b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetUncleByBlockHashAndIndexIntegrationTest.java
@@ -52,7 +52,7 @@ import org.junit.Test;
 
 public class EthGetUncleByBlockHashAndIndexIntegrationTest {
 
-  private static final String CHAIN_ID = "6986785976597";
+  private static final int CHAIN_ID = 123;
   private static JsonRpcTestMethodsFactory BLOCKCHAIN;
 
   private final JsonRpcResponseUtils responseUtils = new JsonRpcResponseUtils();
diff --git a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetUncleByBlockNumberAndIndexIntegrationTest.java b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetUncleByBlockNumberAndIndexIntegrationTest.java
index 6e5ace1e50..46f1f4555c 100644
--- a/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetUncleByBlockNumberAndIndexIntegrationTest.java
+++ b/ethereum/jsonrpc/src/integration-test/java/tech/pegasys/pantheon/ethereum/jsonrpc/methods/EthGetUncleByBlockNumberAndIndexIntegrationTest.java
@@ -52,7 +52,7 @@ import org.junit.Test;
 
 public class EthGetUncleByBlockNumberAndIndexIntegrationTest {
 
-  private static final String CHAIN_ID = "6986785976597";
+  private static final int CHAIN_ID = 123;
   private static JsonRpcTestMethodsFactory BLOCKCHAIN;
 
   private final JsonRpcResponseUtils responseUtils = new JsonRpcResponseUtils();
diff --git a/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcMethodsFactory.java b/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcMethodsFactory.java
index c2e9031ac6..ffb6bf5da9 100644
--- a/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcMethodsFactory.java
+++ b/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcMethodsFactory.java
@@ -24,6 +24,7 @@ import tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods.DebugTraceTransac
 import tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods.EthAccounts;
 import tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods.EthBlockNumber;
 import tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods.EthCall;
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods.EthChainId;
 import tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods.EthCoinbase;
 import tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods.EthEstimateGas;
 import tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods.EthGasPrice;
@@ -87,7 +88,7 @@ public class JsonRpcMethodsFactory {
 
   public Map<String, JsonRpcMethod> methods(
       final String clientVersion,
-      final String chainId,
+      final int chainId,
       final P2PNetwork peerNetworkingService,
       final Blockchain blockchain,
       final WorldStateArchive worldStateArchive,
@@ -116,7 +117,7 @@ public class JsonRpcMethodsFactory {
 
   public Map<String, JsonRpcMethod> methods(
       final String clientVersion,
-      final String chainId,
+      final int chainId,
       final P2PNetwork p2pNetwork,
       final BlockchainQueries blockchainQueries,
       final Synchronizer synchronizer,
@@ -178,7 +179,8 @@ public class JsonRpcMethodsFactory {
           new EthCoinbase(miningCoordinator),
           new EthProtocolVersion(supportedCapabilities),
           new EthGasPrice(miningCoordinator),
-          new EthGetWork(miningCoordinator));
+          new EthGetWork(miningCoordinator),
+          new EthChainId(chainId));
     }
     if (rpcApis.contains(RpcApis.DEBUG)) {
       final BlockReplay blockReplay =
diff --git a/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/EthChainId.java b/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/EthChainId.java
new file mode 100644
index 0000000000..6ed0d39bb2
--- /dev/null
+++ b/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/EthChainId.java
@@ -0,0 +1,37 @@
+/*
+ * Copyright 2018 ConsenSys AG.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+package tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods;
+
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.JsonRpcRequest;
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.response.JsonRpcResponse;
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.response.JsonRpcSuccessResponse;
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.results.Quantity;
+
+public class EthChainId implements JsonRpcMethod {
+
+  private final int chainId;
+
+  public EthChainId(final int chainId) {
+    this.chainId = chainId;
+  }
+
+  @Override
+  public String getName() {
+    return "eth_chainId";
+  }
+
+  @Override
+  public JsonRpcResponse response(final JsonRpcRequest req) {
+    return new JsonRpcSuccessResponse(req.getId(), Quantity.create(chainId));
+  }
+}
diff --git a/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/NetVersion.java b/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/NetVersion.java
index 7646effe78..ca71ca8a3c 100644
--- a/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/NetVersion.java
+++ b/ethereum/jsonrpc/src/main/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/NetVersion.java
@@ -25,8 +25,8 @@ import tech.pegasys.pantheon.ethereum.jsonrpc.internal.response.JsonRpcSuccessRe
 public class NetVersion implements JsonRpcMethod {
   private final String chainId;
 
-  public NetVersion(final String chainId) {
-    this.chainId = chainId;
+  public NetVersion(final int chainId) {
+    this.chainId = String.valueOf(chainId);
   }
 
   @Override
diff --git a/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/AbstractEthJsonRpcHttpServiceTest.java b/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/AbstractEthJsonRpcHttpServiceTest.java
index 6074be9473..cddb041bcd 100644
--- a/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/AbstractEthJsonRpcHttpServiceTest.java
+++ b/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/AbstractEthJsonRpcHttpServiceTest.java
@@ -92,7 +92,7 @@ public abstract class AbstractEthJsonRpcHttpServiceTest {
 
   protected final String CLIENT_VERSION = "TestClientVersion/0.1.0";
 
-  protected final String NET_VERSION = "6986785976597";
+  protected final int NET_VERSION = 123;
 
   protected static final Collection<RpcApi> JSON_RPC_APIS =
       Arrays.asList(RpcApis.ETH, RpcApis.NET, RpcApis.WEB3);
diff --git a/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcHttpServiceRpcApisTest.java b/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcHttpServiceRpcApisTest.java
index 774e374daa..dfbb1be539 100644
--- a/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcHttpServiceRpcApisTest.java
+++ b/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcHttpServiceRpcApisTest.java
@@ -61,7 +61,7 @@ public class JsonRpcHttpServiceRpcApisTest {
   private static String baseUrl;
   private static final MediaType JSON = MediaType.parse("application/json; charset=utf-8");
   private static final String CLIENT_VERSION = "TestClientVersion/0.1.0";
-  private static final String NET_VERSION = "6986785976597";
+  private static final int NET_VERSION = 123;
   private JsonRpcConfiguration configuration;
 
   @Mock protected static BlockchainQueries blockchainQueries;
diff --git a/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcHttpServiceTest.java b/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcHttpServiceTest.java
index ce792c03f9..926132e856 100644
--- a/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcHttpServiceTest.java
+++ b/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/JsonRpcHttpServiceTest.java
@@ -87,7 +87,7 @@ public class JsonRpcHttpServiceTest {
   protected static String baseUrl;
   protected static final MediaType JSON = MediaType.parse("application/json; charset=utf-8");
   protected static final String CLIENT_VERSION = "TestClientVersion/0.1.0";
-  protected static final String NET_VERSION = "6986785976597";
+  protected static final int NET_VERSION = 123;
   protected static P2PNetwork peerDiscoveryMock;
   protected static BlockchainQueries blockchainQueries;
   protected static Synchronizer synchronizer;
@@ -266,7 +266,7 @@ public class JsonRpcHttpServiceTest {
       testHelper.assertValidJsonRpcResult(json, id);
       // Check result
       final String result = json.getString("result");
-      assertThat(result).isEqualTo(NET_VERSION);
+      assertThat(result).isEqualTo(String.valueOf(NET_VERSION));
     }
   }
 
@@ -1544,7 +1544,7 @@ public class JsonRpcHttpServiceTest {
       // Check result net_version
       final JsonObject jsonNetVersion = responses.get(netVersionRequestId);
       testHelper.assertValidJsonRpcResult(jsonNetVersion, netVersionRequestId);
-      assertThat(jsonNetVersion.getString("result")).isEqualTo(NET_VERSION);
+      assertThat(jsonNetVersion.getString("result")).isEqualTo(String.valueOf(NET_VERSION));
     }
   }
 
@@ -1600,7 +1600,7 @@ public class JsonRpcHttpServiceTest {
       // Check result net_version
       final JsonObject jsonNetVersion = responses.get(netVersionRequestId);
       testHelper.assertValidJsonRpcResult(jsonNetVersion, netVersionRequestId);
-      assertThat(jsonNetVersion.getString("result")).isEqualTo(NET_VERSION);
+      assertThat(jsonNetVersion.getString("result")).isEqualTo(String.valueOf(NET_VERSION));
     }
   }
 
@@ -1661,7 +1661,7 @@ public class JsonRpcHttpServiceTest {
       // Check result net_version
       final JsonObject jsonNetVersion = responses.get(netVersionRequestId);
       testHelper.assertValidJsonRpcResult(jsonNetVersion, netVersionRequestId);
-      assertThat(jsonNetVersion.getString("result")).isEqualTo(NET_VERSION);
+      assertThat(jsonNetVersion.getString("result")).isEqualTo(String.valueOf(NET_VERSION));
     }
   }
 
diff --git a/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/EthChainIdTest.java b/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/EthChainIdTest.java
new file mode 100644
index 0000000000..d6bfffd9d1
--- /dev/null
+++ b/ethereum/jsonrpc/src/test/java/tech/pegasys/pantheon/ethereum/jsonrpc/internal/methods/EthChainIdTest.java
@@ -0,0 +1,52 @@
+/*
+ * Copyright 2018 ConsenSys AG.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ * an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations under the License.
+ */
+package tech.pegasys.pantheon.ethereum.jsonrpc.internal.methods;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.JsonRpcRequest;
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.response.JsonRpcResponse;
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.response.JsonRpcSuccessResponse;
+import tech.pegasys.pantheon.ethereum.jsonrpc.internal.results.Quantity;
+
+import org.junit.Before;
+import org.junit.Test;
+
+public class EthChainIdTest {
+
+  private EthChainId method;
+  private final int CHAIN_ID = 1;
+
+  @Before
+  public void setUp() {
+    method = new EthChainId(CHAIN_ID);
+  }
+
+  @Test
+  public void shouldReturnCorrectMethodName() {
+    assertThat(method.getName()).isEqualTo("eth_chainId");
+  }
+
+  @Test
+  public void shouldReturnChainId() {
+    JsonRpcResponse expectedResponse = new JsonRpcSuccessResponse(null, Quantity.create(CHAIN_ID));
+
+    JsonRpcResponse response = method.response(request());
+
+    assertThat(response).isEqualToComparingFieldByField(expectedResponse);
+  }
+
+  private JsonRpcRequest request() {
+    return new JsonRpcRequest(null, "eth_chainId", null);
+  }
+}
diff --git a/pantheon/src/main/java/tech/pegasys/pantheon/RunnerBuilder.java b/pantheon/src/main/java/tech/pegasys/pantheon/RunnerBuilder.java
index b81ede3c6e..dacdaf3e71 100644
--- a/pantheon/src/main/java/tech/pegasys/pantheon/RunnerBuilder.java
+++ b/pantheon/src/main/java/tech/pegasys/pantheon/RunnerBuilder.java
@@ -224,7 +224,7 @@ public class RunnerBuilder {
         new JsonRpcMethodsFactory()
             .methods(
                 PantheonInfo.version(),
-                String.valueOf(pantheonController.getGenesisConfig().getChainId()),
+                pantheonController.getGenesisConfig().getChainId(),
                 networkRunner.getNetwork(),
                 context.getBlockchain(),
                 context.getWorldStateArchive(),