diff --git a/src/Stratis.Bitcoin.Features.SmartContracts.Tests/ApiLogDeserializerTests.cs b/src/Stratis.Bitcoin.Features.SmartContracts.Tests/ApiLogDeserializerTests.cs
index 3d071bd737..33a1aaa86d 100644
--- a/src/Stratis.Bitcoin.Features.SmartContracts.Tests/ApiLogDeserializerTests.cs
+++ b/src/Stratis.Bitcoin.Features.SmartContracts.Tests/ApiLogDeserializerTests.cs
@@ -18,6 +18,8 @@ public struct TestLog
             public byte[] Datas;
             public bool Truth;
             public Address Address;
+            public UInt128 Value128;
+            public UInt256 Value256;
         }
 
         [Fact]
@@ -33,7 +35,9 @@ public void Deserialize_Basic_Log_Success()
                 Data = 0xAA,
                 Datas = new byte[] { 0xBB, 0xCC, 0xDD },
                 Truth = true,
-                Address = "0x0000000000000000000000000000000000000001".HexToAddress()
+                Address = "0x0000000000000000000000000000000000000001".HexToAddress(),
+                Value128 = 123,
+                Value256 = 456
             };
 
             var testBytes = primitiveSerializer.Serialize(testStruct);
@@ -47,6 +51,8 @@ public void Deserialize_Basic_Log_Success()
             Assert.True(testStruct.Datas.SequenceEqual((byte[])deserializedLog.Datas));
             Assert.Equal(testStruct.Truth, deserializedLog.Truth);
             Assert.Equal(testStruct.Address.ToUint160().ToBase58Address(network), deserializedLog.Address);
+            Assert.Equal(testStruct.Value128.ToString(), deserializedLog.Value128.ToString());
+            Assert.Equal(testStruct.Value256.ToString(), deserializedLog.Value256.ToString());
         }
     }
 }
diff --git a/src/Stratis.Bitcoin.Features.SmartContracts/ApiLogDeserializer.cs b/src/Stratis.Bitcoin.Features.SmartContracts/ApiLogDeserializer.cs
index 33db691a55..cd3d829bf2 100644
--- a/src/Stratis.Bitcoin.Features.SmartContracts/ApiLogDeserializer.cs
+++ b/src/Stratis.Bitcoin.Features.SmartContracts/ApiLogDeserializer.cs
@@ -55,6 +55,9 @@ public dynamic DeserializeLogData(byte[] bytes, Type type)
                 {
                     object fieldValue = this.primitiveSerializer.Deserialize(fieldType, fieldBytes);
 
+                    if (fieldType == typeof(UInt128) || fieldType == typeof(UInt256))
+                        fieldValue = fieldValue.ToString();
+
                     instance[field.Name] = fieldValue;
                 }
             }
diff --git a/src/Stratis.Bitcoin.Features.SmartContracts/ReflectionExecutor/ContractParametersContractResolver.cs b/src/Stratis.Bitcoin.Features.SmartContracts/ReflectionExecutor/ContractParametersContractResolver.cs
index dcf80e4e67..3861ea8204 100644
--- a/src/Stratis.Bitcoin.Features.SmartContracts/ReflectionExecutor/ContractParametersContractResolver.cs
+++ b/src/Stratis.Bitcoin.Features.SmartContracts/ReflectionExecutor/ContractParametersContractResolver.cs
@@ -22,6 +22,14 @@ protected override JsonContract CreateContract(Type objectType)
             {
                 contract.Converter = new AddressJsonConverter(this.network);
             }
+            else if (objectType == typeof(UInt128))
+            {
+                contract.Converter = new ToStringJsonConverter<UInt128>();
+            }
+            else if (objectType == typeof(UInt256))
+            {
+                contract.Converter = new ToStringJsonConverter<UInt256>();
+            }
 
             return contract;
         }
diff --git a/src/Stratis.Bitcoin.Features.SmartContracts/ReflectionExecutor/ToStringJsonConverter.cs b/src/Stratis.Bitcoin.Features.SmartContracts/ReflectionExecutor/ToStringJsonConverter.cs
new file mode 100644
index 0000000000..6561dccb0e
--- /dev/null
+++ b/src/Stratis.Bitcoin.Features.SmartContracts/ReflectionExecutor/ToStringJsonConverter.cs
@@ -0,0 +1,36 @@
+using System;
+using NBitcoin;
+using Newtonsoft.Json;
+using Newtonsoft.Json.Linq;
+
+namespace Stratis.Bitcoin.Features.SmartContracts.ReflectionExecutor
+{
+    public class ToStringJsonConverter<T> : JsonConverter
+    {
+        public ToStringJsonConverter()
+        {
+        }
+
+        public override bool CanRead
+        {
+            get { return false; }
+        }
+
+        public override bool CanConvert(Type objectType)
+        {
+            return objectType == typeof(T);
+        }
+
+        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
+        {
+            throw new NotImplementedException("Unnecessary because CanRead is false. The type will skip the converter.");
+        }
+
+        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
+        {
+            JToken t = JToken.FromObject(value);
+            JValue v = JValue.CreateString(value.ToString());
+            v.WriteTo(writer);
+        }
+    }
+}
diff --git a/src/Stratis.SmartContracts.CLR.Tests/ContractParametersJsonResolverTests.cs b/src/Stratis.SmartContracts.CLR.Tests/ContractParametersJsonResolverTests.cs
index 1d2cba7258..17d6199f00 100644
--- a/src/Stratis.SmartContracts.CLR.Tests/ContractParametersJsonResolverTests.cs
+++ b/src/Stratis.SmartContracts.CLR.Tests/ContractParametersJsonResolverTests.cs
@@ -33,6 +33,32 @@ public void Address_Json_Outputs_As_Base58String()
             Assert.Equal(expectedString, jsonOutput.Replace("\"", ""));
         }
 
+        [Fact]
+        public void UInt256_Json_Outputs_As_String()
+        {
+            UInt256 testUInt256 = (UInt256)123;
+            string expectedString = testUInt256.ToString();
+
+            string jsonOutput = JsonConvert.SerializeObject(testUInt256, new JsonSerializerSettings
+            {
+                ContractResolver = this.resolver
+            });
+            Assert.Equal(expectedString, jsonOutput.Replace("\"", ""));
+        }
+        
+        [Fact]
+        public void UInt128_Json_Outputs_As_String()
+        {
+            UInt128 testUInt128 = (UInt128)123;
+            string expectedString = testUInt128.ToString();
+
+            string jsonOutput = JsonConvert.SerializeObject(testUInt128, new JsonSerializerSettings
+            {
+                ContractResolver = this.resolver
+            });
+            Assert.Equal(expectedString, jsonOutput.Replace("\"", ""));
+        }
+
         [Fact]
         public void LocalExecutionResult_Outputs_With_Address()
         {