diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index fb34312..94ea338 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -1,6 +1,13 @@
 # Release Notes
 
-## XMLUnit.NET 2.8.1 - /not released, yet/
+## XMLUnit.NET 2.9.0 - /not released, yet/
+
+* `ISource` now extends `IDisposable` to allow releasing unmanaged
+  resources used when building sources from files or URIs.
+
+  This change is backwards incompatible if you are providing `ISource`
+  implementations of your own.
+  [#33](https://github.com/xmlunit/xmlunit.net/pull/33).
 
 ## XMLUnit.NET 2.8.0 - /Released 2020-05-12/
 
diff --git a/appveyor.yml b/appveyor.yml
index 31a3ac4..e054d41 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,4 +1,4 @@
-version: 2.8.1.{build}
+version: 2.9.0.{build}
 
 image:
   - Visual Studio 2013
diff --git a/src/doc/monodoc/core/index.xml b/src/doc/monodoc/core/index.xml
index 1f718e0..32eb6f8 100644
--- a/src/doc/monodoc/core/index.xml
+++ b/src/doc/monodoc/core/index.xml
@@ -45,5 +45,5 @@
       under <format type="text/html"><a href="https://github.com/xmlunit/xmlunit.net/blob/main/LICENSE">the
       Apache License, Version 2.0</a></format>.</para></Remarks>
   <Copyright></Copyright>
-  <Title>XMLUnit.NET 2.8.1-alpha-01</Title>
+  <Title>XMLUnit.NET 2.9.0-alpha-01</Title>
 </Overview>
diff --git a/src/main/net-core/ISource.cs b/src/main/net-core/ISource.cs
index 58b856d..ca08922 100644
--- a/src/main/net-core/ISource.cs
+++ b/src/main/net-core/ISource.cs
@@ -18,7 +18,7 @@ namespace Org.XmlUnit {
     /// Representation of the various ways to provide pieces of XML to
     /// XMLUnit.
     /// </summary>
-    public interface ISource {
+    public interface ISource : System.IDisposable {
         /// <summary>
         /// Provides the content.
         /// </summary>
diff --git a/src/main/net-core/Input/AbstractSource.cs b/src/main/net-core/Input/AbstractSource.cs
index 0c7918d..8ed502f 100644
--- a/src/main/net-core/Input/AbstractSource.cs
+++ b/src/main/net-core/Input/AbstractSource.cs
@@ -21,6 +21,7 @@ namespace Org.XmlUnit.Input
     /// </summary>
     public abstract class AbstractSource : ISource
     {
+        private bool disposed;
         private string systemId;
         private readonly XmlReader reader;
         /// <summary>
@@ -61,5 +62,16 @@ public override string ToString()
             return string.Format("{0} with systemId {1}", GetType().Name,
                                  SystemId);
         }
+
+        public void Dispose() {
+            Dispose(false);
+        }
+
+        protected virtual void Dispose(bool disposing) {
+            if (!disposed) {
+                reader.Close();
+                disposed = true;
+            }
+        }
     }
 }
diff --git a/src/main/net-core/Input/ByteArraySource.cs b/src/main/net-core/Input/ByteArraySource.cs
index 8b36a61..9855505 100644
--- a/src/main/net-core/Input/ByteArraySource.cs
+++ b/src/main/net-core/Input/ByteArraySource.cs
@@ -59,5 +59,7 @@ public override string ToString()
         {
             return string.Format("ByteArraySource with systemId {0}", SystemId);
         }
+
+        public void Dispose() { }
     }
 }
diff --git a/src/main/net-core/Input/CommentLessSource.cs b/src/main/net-core/Input/CommentLessSource.cs
index 5bae3a8..4986982 100644
--- a/src/main/net-core/Input/CommentLessSource.cs
+++ b/src/main/net-core/Input/CommentLessSource.cs
@@ -31,6 +31,7 @@ namespace Org.XmlUnit.Input {
     ///   </para>
     /// </remarks>
     public sealed class CommentLessSource : ISource {
+        private bool disposed;
         private readonly XmlReader reader;
         private string systemId;
 
@@ -100,6 +101,13 @@ public string SystemId
             }
         }
 
+        public void Dispose() {
+            if (!disposed) {
+                reader.Close();
+                disposed = true;
+            }
+        }
+
         private static ISource GetStylesheet(string xsltVersion) {
             return new StreamSource(new System.IO.StringReader(GetStylesheetContentCached(xsltVersion)));
         }
diff --git a/src/main/net-core/Input/NormalizedSource.cs b/src/main/net-core/Input/NormalizedSource.cs
index 4d50a64..5f14e5c 100644
--- a/src/main/net-core/Input/NormalizedSource.cs
+++ b/src/main/net-core/Input/NormalizedSource.cs
@@ -36,6 +36,7 @@ namespace Org.XmlUnit.Input {
     ///   </para>
     /// </remarks>
     public class NormalizedSource : ISource {
+        private bool disposed;
         private string systemId;
         private readonly XmlReader reader;
         private readonly XmlNode node;
@@ -107,5 +108,16 @@ public override string ToString()
             return string.Format("{0} with systemId {1}", GetType().Name,
                                  SystemId);
         }
+
+        public void Dispose() {
+            Dispose(false);
+        }
+
+        protected virtual void Dispose(bool disposing) {
+            if (!disposed) {
+                reader.Close();
+                disposed = true;
+            }
+        }
     }
 }
diff --git a/src/main/net-core/Input/StringSource.cs b/src/main/net-core/Input/StringSource.cs
index bc28990..7f1f625 100644
--- a/src/main/net-core/Input/StringSource.cs
+++ b/src/main/net-core/Input/StringSource.cs
@@ -60,5 +60,7 @@ public override string ToString()
             return string.Format("StringSource with content{0} and systemId {1}", content,
                                  SystemId);
         }
+
+        public void Dispose() { }
     }
 }
diff --git a/src/shared/CommonAssemblyInfo.cs b/src/shared/CommonAssemblyInfo.cs
index 8a4ef47..fc63347 100644
--- a/src/shared/CommonAssemblyInfo.cs
+++ b/src/shared/CommonAssemblyInfo.cs
@@ -25,7 +25,7 @@ namespace Org.XmlUnit
 {
     internal static class XmlUnitVersion
     {
-        internal const string ApiVersion = "2.8.1";
+        internal const string ApiVersion = "2.9.0";
         internal const string AssemblyVersion = ApiVersion + ".198";
         internal const string Version = ApiVersion + "-alpha-01";
     }