diff --git a/CHANGELOG.md b/CHANGELOG.md
index e97987f4..48debfd2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,7 +8,8 @@ Represents the **NuGet** versions.
 - *Enhancement*: Upgraded `UnitTestEx` dependency to `4.0.2` to enable _isolated_ function testing.
 - *Enhancement*: Enabled `IJsonSerializer` support for `CompositeKey` JSON serialization/deserialization.
 - *Enhancement*: Added `IEventDataFormatter` which when implemented by the value set as the `EventData.Value` allows additional formatting to be applied by the `EventDataFormatter`.
-- *Fixed*: `EventDataFormatter` and `CloudEventSerializerBase` updated to correctly set the `Key` where applicable.
+- *Fixed*: Added `ReferenceDataMultiDictionaryConverterFactory` to ensure each `IReferenceDataCollection` is serialized correctly according to its underlying type.
+- *Fixed*: `EventDataFormatter` and `CloudEventSerializerBase` updated to correctly set the `Key` property where applicable.
 - *Internal:* Upgraded `NUnit` dependency to `4.0.1` for all `CoreEx` unit test; also, all unit tests now leverage the [_NUnit constraint model_](https://docs.nunit.org/articles/nunit/writing-tests/assertions/assertion-models/constraint.html) testing approach.
 
 ## v3.8.1
diff --git a/samples/My.Hr/My.Hr.UnitTest/ReferenceDataControllerTest.cs b/samples/My.Hr/My.Hr.UnitTest/ReferenceDataControllerTest.cs
index 76509784..f9cdde52 100644
--- a/samples/My.Hr/My.Hr.UnitTest/ReferenceDataControllerTest.cs
+++ b/samples/My.Hr/My.Hr.UnitTest/ReferenceDataControllerTest.cs
@@ -94,7 +94,7 @@ public void C100_Named()
             using var test = ApiTester.Create<Startup>().UseJsonSerializer(new ReferenceDataContentJsonSerializer().ToUnitTestEx());
 
             var r = test.Controller<ReferenceDataController>()
-                .Run(c => c.GetNamed(), new HttpRequestOptions { UrlQueryString = "gender&usstate" })
+                .Run(c => c.GetNamed(), requestOptions: new HttpRequestOptions { UrlQueryString = "gender&usstate" })
                 .AssertOK();
         }
     }
diff --git a/src/CoreEx/Text/Json/ReferenceDataContentJsonSerializer.cs b/src/CoreEx/Text/Json/ReferenceDataContentJsonSerializer.cs
index 41f0b3e5..ce5acc5a 100644
--- a/src/CoreEx/Text/Json/ReferenceDataContentJsonSerializer.cs
+++ b/src/CoreEx/Text/Json/ReferenceDataContentJsonSerializer.cs
@@ -23,7 +23,7 @@ public class ReferenceDataContentJsonSerializer(Stj.JsonSerializerOptions? optio
         ///  <item><description><see cref="Stj.JsonSerializerOptions.WriteIndented"/> = <c>false</c></description></item>
         ///  <item><description><see cref="Stj.JsonSerializerOptions.DictionaryKeyPolicy"/> = <see cref="SubstituteNamingPolicy.Substitute"/>.</description></item>
         ///  <item><description><see cref="Stj.JsonSerializerOptions.PropertyNamingPolicy"/> = <see cref="SubstituteNamingPolicy.Substitute"/>.</description></item>
-        ///  <item><description><see cref="Stj.JsonSerializerOptions.Converters"/> = <see cref="JsonStringEnumConverter"/>, <see cref="ExceptionConverterFactory"/>, <see cref="CollectionResultConverterFactory"/> and <see cref="ResultConverterFactory"/>.</description></item>
+        ///  <item><description><see cref="Stj.JsonSerializerOptions.Converters"/> = <see cref="JsonStringEnumConverter"/>, <see cref="ExceptionConverterFactory"/>, <see cref="CollectionResultConverterFactory"/>, <see cref="ResultConverterFactory"/> and <see cref="ReferenceDataMultiDictionaryConverterFactory"/>.</description></item>
         /// </list>
         /// </remarks>
         public static new Stj.JsonSerializerOptions DefaultOptions { get; set; } = new Stj.JsonSerializerOptions(Stj.JsonSerializerDefaults.Web)
@@ -32,7 +32,7 @@ public class ReferenceDataContentJsonSerializer(Stj.JsonSerializerOptions? optio
             WriteIndented = false,
             DictionaryKeyPolicy = SubstituteNamingPolicy.Substitute,
             PropertyNamingPolicy = SubstituteNamingPolicy.Substitute,
-            Converters = { new JsonStringEnumConverter(), new ExceptionConverterFactory(), new CollectionResultConverterFactory(), new ResultConverterFactory() }
+            Converters = { new JsonStringEnumConverter(), new ExceptionConverterFactory(), new CollectionResultConverterFactory(), new ResultConverterFactory(), new ReferenceDataMultiDictionaryConverterFactory() }
         };
     }
 }
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diff --git a/src/CoreEx/Text/Json/ReferenceDataMultiDictionaryConverterFactory.cs b/src/CoreEx/Text/Json/ReferenceDataMultiDictionaryConverterFactory.cs
new file mode 100644
index 00000000..b726a23b
--- /dev/null
+++ b/src/CoreEx/Text/Json/ReferenceDataMultiDictionaryConverterFactory.cs
@@ -0,0 +1,45 @@
+// Copyright (c) Avanade. Licensed under the MIT License. See https://github.com/Avanade/CoreEx
+
+using CoreEx.RefData;
+using System;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace CoreEx.Text.Json
+{
+    /// <summary>
+    /// Performs JSON value conversion for <see cref="ReferenceDataMultiDictionary"/> values.
+    /// </summary>
+    /// <remarks>This is required to ensure each <see cref="IReferenceDataCollection"/> is serialized correctly according to its underlying type.</remarks>
+    public class ReferenceDataMultiDictionaryConverterFactory : JsonConverterFactory
+    {
+        /// <inheritdoc/>
+        public override bool CanConvert(Type typeToConvert) => typeToConvert == typeof(ReferenceDataMultiDictionary);
+
+        /// <inheritdoc/>
+        public override JsonConverter? CreateConverter(Type typeToConvert, JsonSerializerOptions options) => new ReferenceDataMultiDictionaryConverter();
+
+        /// <summary>
+        /// Performs the "actual" JSON value conversion for <see cref="ReferenceDataMultiDictionary"/> values.
+        /// </summary>
+        private class ReferenceDataMultiDictionaryConverter : JsonConverter<ReferenceDataMultiDictionary>
+        {
+            /// <inheritdoc/>
+            public override ReferenceDataMultiDictionary Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) => throw new NotSupportedException($"Deserialization of Type {nameof(ReferenceDataMultiDictionary)} is not supported.");
+
+            /// <inheritdoc/>
+            public override void Write(Utf8JsonWriter writer, ReferenceDataMultiDictionary value, JsonSerializerOptions options)
+            {
+                writer.WriteStartObject();
+
+                foreach (var kvp in value)
+                {
+                    writer.WritePropertyName(options.DictionaryKeyPolicy?.ConvertName(kvp.Key) ?? kvp.Key);
+                    System.Text.Json.JsonSerializer.Serialize(writer, kvp.Value, kvp.Value.GetType(), options);
+                }
+
+                writer.WriteEndObject();
+            }
+        }
+    }
+}
\ No newline at end of file