diff --git a/Doc/library/platform.rst b/Doc/library/platform.rst
--- a/Doc/library/platform.rst
+++ b/Doc/library/platform.rst
@@ -208,7 +208,7 @@
 Win95/98 specific
 ^^^^^^^^^^^^^^^^^
 
-.. function:: popen(cmd, mode='r', bufsize=None)
+.. function:: popen(cmd, mode='r', bufsize=1)
 
    Portable :func:`popen` interface.  Find a working popen implementation
    preferring :func:`win32pipe.popen`.  On Windows NT, :func:`win32pipe.popen`
diff --git a/Lib/os.py b/Lib/os.py
--- a/Lib/os.py
+++ b/Lib/os.py
@@ -779,11 +779,13 @@
         return bs
 
 # Supply os.popen()
-def popen(cmd, mode="r", buffering=None):
+def popen(cmd, mode="r", buffering=1):
     if not isinstance(cmd, str):
         raise TypeError("invalid cmd type (%s, expected string)" % type(cmd))
     if mode not in ("r", "w"):
         raise ValueError("invalid mode %r" % mode)
+    if buffering == 0 or buffering == None:
+        raise ValueError("popen() does not support unbuffered streams")
     import subprocess, io
     if mode == "r":
         proc = subprocess.Popen(cmd,
diff --git a/Lib/platform.py b/Lib/platform.py
--- a/Lib/platform.py
+++ b/Lib/platform.py
@@ -357,7 +357,7 @@
                               supported_dists=supported_dists,
                               full_distribution_name=0)
 
-def popen(cmd, mode='r', bufsize=None):
+def popen(cmd, mode='r', bufsize=1):
 
     """ Portable popen() interface.
     """
diff --git a/Lib/subprocess.py b/Lib/subprocess.py
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -741,8 +741,6 @@
             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: