diff --git a/src/Common/src/CoreLib/System/Text/DecoderReplacementFallback.cs b/src/Common/src/CoreLib/System/Text/DecoderReplacementFallback.cs
index 1dc41d8d229b..7147b3ea0a28 100644
--- a/src/Common/src/CoreLib/System/Text/DecoderReplacementFallback.cs
+++ b/src/Common/src/CoreLib/System/Text/DecoderReplacementFallback.cs
@@ -3,13 +3,16 @@
 // See the LICENSE file in the project root for more information.
 
 using System.Diagnostics;
+using System.Runtime.Serialization;
 
 namespace System.Text
 {
 #if MONO
     [System.Serializable]
-#endif
+    public sealed class DecoderReplacementFallback : DecoderFallback, ISerializable
+#else
     public sealed class DecoderReplacementFallback : DecoderFallback
+#endif
     {
         // Our variables
         private String _strDefault;
@@ -19,6 +22,16 @@ public DecoderReplacementFallback() : this("?")
         {
         }
 
+#if MONO
+        internal DecoderReplacementFallback(SerializationInfo info, StreamingContext context)
+        {
+            try { _strDefault = info.GetString("strDefault"); } // for old mono and .NET 4.x
+            catch { _strDefault = info.GetString("_strDefault"); }
+        }
+
+        void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context) => info.AddValue("strDefault", _strDefault);
+#endif
+
         public DecoderReplacementFallback(String replacement)
         {
             if (replacement == null)
diff --git a/src/Common/src/CoreLib/System/Text/EncoderReplacementFallback.cs b/src/Common/src/CoreLib/System/Text/EncoderReplacementFallback.cs
index ff97fbaf41e0..eac4c9e3a8cd 100644
--- a/src/Common/src/CoreLib/System/Text/EncoderReplacementFallback.cs
+++ b/src/Common/src/CoreLib/System/Text/EncoderReplacementFallback.cs
@@ -5,13 +5,16 @@
 using System;
 using System.Runtime;
 using System.Diagnostics;
+using System.Runtime.Serialization;
 
 namespace System.Text
 {
 #if MONO
     [Serializable]
-#endif
+    public sealed class EncoderReplacementFallback : EncoderFallback, ISerializable
+#else
     public sealed class EncoderReplacementFallback : EncoderFallback
+#endif
     {
         // Our variables
         private String _strDefault;
@@ -21,6 +24,16 @@ public EncoderReplacementFallback() : this("?")
         {
         }
 
+#if MONO
+        internal EncoderReplacementFallback(SerializationInfo info, StreamingContext context)
+        {
+            try { _strDefault = info.GetString("strDefault"); } // for old mono and .NET 4.x
+            catch { _strDefault = info.GetString("_strDefault"); }
+        }
+
+        void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context) => info.AddValue("strDefault", _strDefault);
+#endif
+
         public EncoderReplacementFallback(String replacement)
         {
             // Must not be null