Index: Lib/subprocess.py
===================================================================
--- Lib/subprocess.py	(revision 85719)
+++ Lib/subprocess.py	(working copy)
@@ -617,6 +617,8 @@
             bufsize = 0  # Restore default
         if not isinstance(bufsize, int):
             raise TypeError("bufsize must be an integer")
+        if bufsize == 0 and universal_newlines:
+            raise ValueError("cannot use bufsize=0 with universal newlines")
 
         if mswindows:
             if preexec_fn is not None:
@@ -677,8 +679,6 @@
             if errread != -1:
                 errread = msvcrt.open_osfhandle(errread.Detach(), 0)
 
-        if bufsize == 0:
-            bufsize = 1  # Nearly unbuffered (XXX for now)
         if p2cwrite != -1:
             self.stdin = io.open(p2cwrite, 'wb', bufsize)
             if self.universal_newlines:
Index: Lib/test/test_subprocess.py
===================================================================
--- Lib/test/test_subprocess.py	(revision 85719)
+++ Lib/test/test_subprocess.py	(working copy)
@@ -523,6 +523,20 @@
         self.assertEqual(p.wait(), 0)
 
 
+    def test_bufsize_is_zero(self):
+        # bufsize=0 makes Popen unbuffered
+        p = subprocess.Popen([sys.executable, "-c",
+                             'import sys; sys.exit(sys.stdin.read(1) == "X")'],
+                             stdin=subprocess.PIPE, bufsize=0)
+        p.stdin.write(b"X")
+        p.wait()
+        self.assertEqual(p.returncode, 1)
+        # cannot use bufsize=0 with universal new lines
+        with self.assertRaises(ValueError):
+            subprocess.Popen([sys.executable, "-c", "pass"],
+                             stdin=subprocess.PIPE, bufsize=0,
+                             universal_newlines=True)
+
     def test_invalid_bufsize(self):
         # an invalid type of the bufsize argument should raise
         # TypeError.