diff --git a/src/System.Net.Http.Formatting/Formatting/FormUrlEncodedMediaTypeFormatter.cs b/src/System.Net.Http.Formatting/Formatting/FormUrlEncodedMediaTypeFormatter.cs
index 0a38a0c4..4b23d9a2 100644
--- a/src/System.Net.Http.Formatting/Formatting/FormUrlEncodedMediaTypeFormatter.cs
+++ b/src/System.Net.Http.Formatting/Formatting/FormUrlEncodedMediaTypeFormatter.cs
@@ -18,11 +18,6 @@ public class FormUrlEncodedMediaTypeFormatter : MediaTypeFormatter
         private const int MinBufferSize = 256;
         private const int DefaultBufferSize = 32 * 1024;
 
-        private static readonly MediaTypeHeaderValue[] _supportedMediaTypes = new MediaTypeHeaderValue[]
-        {
-            MediaTypeConstants.ApplicationFormUrlEncodedMediaType
-        };
-
         private int _readBufferSize = DefaultBufferSize;
         private int _maxDepth = FormattingUtilities.DefaultMaxDepth;
 
@@ -31,10 +26,7 @@ public class FormUrlEncodedMediaTypeFormatter : MediaTypeFormatter
         /// </summary>
         public FormUrlEncodedMediaTypeFormatter()
         {
-            foreach (MediaTypeHeaderValue value in _supportedMediaTypes)
-            {
-                SupportedMediaTypes.Add(value);
-            }
+            SupportedMediaTypes.Add(MediaTypeConstants.ApplicationFormUrlEncodedMediaType);
         }
 
         /// <summary>
diff --git a/src/System.Net.Http.Formatting/Formatting/JsonMediaTypeFormatter.cs b/src/System.Net.Http.Formatting/Formatting/JsonMediaTypeFormatter.cs
index 5f7a9ea7..d95d49b0 100644
--- a/src/System.Net.Http.Formatting/Formatting/JsonMediaTypeFormatter.cs
+++ b/src/System.Net.Http.Formatting/Formatting/JsonMediaTypeFormatter.cs
@@ -22,12 +22,6 @@ namespace System.Net.Http.Formatting
     /// </summary>
     public class JsonMediaTypeFormatter : MediaTypeFormatter
     {
-        private static readonly MediaTypeHeaderValue[] _supportedMediaTypes = new MediaTypeHeaderValue[]
-        {
-            MediaTypeConstants.ApplicationJsonMediaType,
-            MediaTypeConstants.TextJsonMediaType
-        };
-
         private JsonSerializerSettings _jsonSerializerSettings;
         private readonly IContractResolver _defaultContractResolver;
         private int _maxDepth = FormattingUtilities.DefaultMaxDepth;
@@ -42,10 +36,8 @@ public class JsonMediaTypeFormatter : MediaTypeFormatter
         public JsonMediaTypeFormatter()
         {
             // Set default supported media types
-            foreach (MediaTypeHeaderValue value in _supportedMediaTypes)
-            {
-                SupportedMediaTypes.Add(value);
-            }
+            SupportedMediaTypes.Add(MediaTypeConstants.ApplicationJsonMediaType);
+            SupportedMediaTypes.Add(MediaTypeConstants.TextJsonMediaType);
 
             // Initialize serializer
             _defaultContractResolver = new JsonContractResolver(this);
diff --git a/src/System.Net.Http.Formatting/Formatting/XmlMediaTypeFormatter.cs b/src/System.Net.Http.Formatting/Formatting/XmlMediaTypeFormatter.cs
index db3692e5..6e216387 100644
--- a/src/System.Net.Http.Formatting/Formatting/XmlMediaTypeFormatter.cs
+++ b/src/System.Net.Http.Formatting/Formatting/XmlMediaTypeFormatter.cs
@@ -21,12 +21,6 @@ namespace System.Net.Http.Formatting
     /// </summary>
     public class XmlMediaTypeFormatter : MediaTypeFormatter
     {
-        private static readonly MediaTypeHeaderValue[] _supportedMediaTypes = new MediaTypeHeaderValue[]
-        {
-            MediaTypeConstants.ApplicationXmlMediaType,
-            MediaTypeConstants.TextXmlMediaType
-        };
-
         private ConcurrentDictionary<Type, object> _serializerCache = new ConcurrentDictionary<Type, object>();
         private XmlDictionaryReaderQuotas _readerQuotas = FormattingUtilities.CreateDefaultReaderQuotas();
 
@@ -36,10 +30,8 @@ public class XmlMediaTypeFormatter : MediaTypeFormatter
         public XmlMediaTypeFormatter()
         {
             // Set default supported media types
-            foreach (MediaTypeHeaderValue value in _supportedMediaTypes)
-            {
-                SupportedMediaTypes.Add(value);
-            }
+            SupportedMediaTypes.Add(MediaTypeConstants.ApplicationXmlMediaType);
+            SupportedMediaTypes.Add(MediaTypeConstants.TextXmlMediaType);
 
             // Set default supported character encodings
             SupportedEncodings.Add(new UTF8Encoding(encoderShouldEmitUTF8Identifier: false, throwOnInvalidBytes: true));
diff --git a/test/System.Net.Http.Formatting.Test.Unit/Formatting/FormUrlEncodedMediaTypeFormatterTests.cs b/test/System.Net.Http.Formatting.Test.Unit/Formatting/FormUrlEncodedMediaTypeFormatterTests.cs
index 318dcc64..cecc0bc0 100644
--- a/test/System.Net.Http.Formatting.Test.Unit/Formatting/FormUrlEncodedMediaTypeFormatterTests.cs
+++ b/test/System.Net.Http.Formatting.Test.Unit/Formatting/FormUrlEncodedMediaTypeFormatterTests.cs
@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved. See License.txt in the project root for license information.
 
 using System.IO;
+using System.Linq;
 using System.Net.Http.Formatting.DataSets;
 using System.Net.Http.Headers;
 using System.Text;
@@ -25,6 +26,32 @@ public void TypeIsCorrect()
             Assert.Type.HasProperties(typeof(FormUrlEncodedMediaTypeFormatter), TypeAssert.TypeProperties.IsPublicVisibleClass);
         }
 
+        [Fact]
+        public void SupportedMediaTypes_HeaderValuesAreNotSharedBetweenInstances()
+        {
+            var formatter1 = new FormUrlEncodedMediaTypeFormatter();
+            var formatter2 = new FormUrlEncodedMediaTypeFormatter();
+
+            foreach (MediaTypeHeaderValue mediaType1 in formatter1.SupportedMediaTypes)
+            {
+                MediaTypeHeaderValue mediaType2 = formatter2.SupportedMediaTypes.Single(m => m.Equals(mediaType1));
+                Assert.NotSame(mediaType1, mediaType2);
+            }
+        }
+
+        [Fact]
+        public void SupportEncodings_ValuesAreNotSharedBetweenInstances()
+        {
+            var formatter1 = new FormUrlEncodedMediaTypeFormatter();
+            var formatter2 = new FormUrlEncodedMediaTypeFormatter();
+
+            foreach (Encoding encoding1 in formatter1.SupportedEncodings)
+            {
+                Encoding encoding2 = formatter2.SupportedEncodings.Single(e => e.Equals(encoding1));
+                Assert.NotSame(encoding1, encoding2);
+            }
+        }
+
         [Theory]
         [TestDataSet(typeof(HttpUnitTestDataSets), "StandardFormUrlEncodedMediaTypes")]
         [Trait("Description", "FormUrlEncodedMediaTypeFormatter() constructor sets standard form URL encoded media types in SupportedMediaTypes.")]
diff --git a/test/System.Net.Http.Formatting.Test.Unit/Formatting/MediaTypeFormatterTestBase.cs b/test/System.Net.Http.Formatting.Test.Unit/Formatting/MediaTypeFormatterTestBase.cs
index ffba78da..a43ef14e 100644
--- a/test/System.Net.Http.Formatting.Test.Unit/Formatting/MediaTypeFormatterTestBase.cs
+++ b/test/System.Net.Http.Formatting.Test.Unit/Formatting/MediaTypeFormatterTestBase.cs
@@ -41,6 +41,32 @@ public void TypeIsCorrect()
             Assert.Type.HasProperties<TFormatter, MediaTypeFormatter>(TypeAssert.TypeProperties.IsPublicVisibleClass);
         }
 
+        [Fact]
+        public void SupportedMediaTypes_HeaderValuesAreNotSharedBetweenInstances()
+        {
+            var formatter1 = new TFormatter();
+            var formatter2 = new TFormatter();
+
+            foreach (MediaTypeHeaderValue mediaType1 in formatter1.SupportedMediaTypes)
+            {
+                MediaTypeHeaderValue mediaType2 = formatter2.SupportedMediaTypes.Single(m => m.Equals(mediaType1));
+                Assert.NotSame(mediaType1, mediaType2);
+            }
+        }
+
+        [Fact]
+        public void SupportEncodings_ValuesAreNotSharedBetweenInstances()
+        {
+            var formatter1 = new TFormatter();
+            var formatter2 = new TFormatter();
+
+            foreach (Encoding mediaType1 in formatter1.SupportedEncodings)
+            {
+                Encoding mediaType2 = formatter2.SupportedEncodings.Single(m => m.Equals(mediaType1));
+                Assert.NotSame(mediaType1, mediaType2);
+            }
+        }
+
         [Fact]
         public void SupportEncoding_DefaultSupportedMediaTypes()
         {
@@ -60,7 +86,7 @@ public void ReadFromStreamAsync_ThrowsOnNull()
         {
             TFormatter formatter = new TFormatter();
             Assert.ThrowsArgumentNull(() => { formatter.ReadFromStreamAsync(null, Stream.Null, null, null); }, "type");
-            Assert.ThrowsArgumentNull(() => { formatter.WriteToStreamAsync(typeof(object), null, null, null, null); }, "stream");
+            Assert.ThrowsArgumentNull(() => { formatter.ReadFromStreamAsync(typeof(object), null, null, null); }, "stream");
         }
 
         [Fact]