diff --git a/Source/cmGeneratedFileStream.cxx b/Source/cmGeneratedFileStream.cxx
index 9cee0e6e96..119067effb 100644
--- a/Source/cmGeneratedFileStream.cxx
+++ b/Source/cmGeneratedFileStream.cxx
@@ -122,10 +122,15 @@ void cmGeneratedFileStreamBase::Open(std::string const& name)
   // Create the name of the temporary file.
   this->TempName = name;
 #if defined(__VMS)
-  this->TempName += "_tmp";
+  this->TempName += "_";
 #else
-  this->TempName += ".tmp";
+  this->TempName += ".";
 #endif
+  if (!this->TempExt.empty()) {
+    this->TempName += this->TempExt;
+  } else {
+    this->TempName += "tmp";
+  }
 
   // Make sure the temporary file that will be used is not present.
   cmSystemTools::RemoveFile(this->TempName);
@@ -216,3 +221,8 @@ void cmGeneratedFileStream::SetName(const std::string& fname)
 {
   this->Name = fname;
 }
+
+void cmGeneratedFileStream::SetTempExt(std::string const& ext)
+{
+  this->TempExt = ext;
+}
diff --git a/Source/cmGeneratedFileStream.h b/Source/cmGeneratedFileStream.h
index a9088ac8af..3dee1429e7 100644
--- a/Source/cmGeneratedFileStream.h
+++ b/Source/cmGeneratedFileStream.h
@@ -43,6 +43,9 @@ protected:
   // The name of the final destination file for the output.
   std::string Name;
 
+  // The extension of the temporary file.
+  std::string TempExt;
+
   // The name of the temporary file.
   std::string TempName;
 
@@ -138,6 +141,12 @@ public:
    * the output file to be changed during the use of cmGeneratedFileStream.
    */
   void SetName(const std::string& fname);
+
+  /**
+   * Set set a custom temporary file extension used with 'Open'.
+   * This does not work if the file was opened by the constructor.
+   */
+  void SetTempExt(std::string const& ext);
 };
 
 #endif