diff -r d9a3d23cf8f0 Lib/email/headerregistry.py
--- a/Lib/email/headerregistry.py	Tue Oct 14 00:53:13 2014 +0200
+++ b/Lib/email/headerregistry.py	Wed Oct 15 10:46:22 2014 +0100
@@ -7,6 +7,7 @@
 and will probably change some before that happens.
 
 """
+from types import MappingProxyType
 
 from email import utils
 from email import errors
@@ -456,7 +457,7 @@
 
     @property
     def params(self):
-        return self._params.copy()
+        return MappingProxyType(self._params)
 
 
 class ContentTypeHeader(ParameterizedMIMEHeader):
diff -r d9a3d23cf8f0 Lib/test/test_email/test_headerregistry.py
--- a/Lib/test/test_email/test_headerregistry.py	Tue Oct 14 00:53:13 2014 +0200
+++ b/Lib/test/test_email/test_headerregistry.py	Wed Oct 15 10:46:22 2014 +0100
@@ -1,6 +1,7 @@
 import datetime
 import textwrap
 import unittest
+import types
 from email import errors
 from email import policy
 from email.message import Message
@@ -235,6 +236,7 @@
         self.assertEqual(h.maintype, maintype)
         self.assertEqual(h.subtype, subtype)
         self.assertEqual(h.params, parmdict)
+        self.assertIsInstance(h.params, types.MappingProxyType)
         self.assertDefectsEqual(h.defects, defects)
         self.assertEqual(h, decoded)
         self.assertEqual(h.fold(policy=policy.default), folded)