diff -r f4754416a726 Lib/plistlib.py
--- a/Lib/plistlib.py	Fri Sep 30 10:38:31 2016 +0300
+++ b/Lib/plistlib.py	Sat Oct 01 09:32:50 2016 +0300
@@ -918,7 +918,7 @@ class _BinaryPlistWriter (object):
                 self._write_size(0x50, len(value))
             except UnicodeEncodeError:
                 t = value.encode('utf-16be')
-                self._write_size(0x60, len(value))
+                self._write_size(0x60, len(t) // 2)
 
             self._fp.write(t)
 
diff -r f4754416a726 Lib/test/test_plistlib.py
--- a/Lib/test/test_plistlib.py	Fri Sep 30 10:38:31 2016 +0300
+++ b/Lib/test/test_plistlib.py	Sat Oct 01 09:32:50 2016 +0300
@@ -361,6 +361,13 @@ class TestPlistlib(unittest.TestCase):
                                   plistlib.dumps,
                                   testString)
 
+    def test_astralcharacters(self):
+        pl = {'python': '\U0001f40d'}
+        for fmt in ALL_FORMATS:
+            with self.subTest(fmt=fmt):
+                data = plistlib.dumps(pl, fmt=fmt)
+                self.assertEqual(plistlib.loads(data), pl)
+
     def test_nondictroot(self):
         for fmt in ALL_FORMATS:
             with self.subTest(fmt=fmt):