diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 560161d91..f6ef60bea 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -5,6 +5,7 @@ Alexandre Conrad
 Allan Feldman
 Andrii Soldatenko
 Anthon van der Neuth
+Anthony Sottile
 Asmund Grammeltwedt
 Barry Warsaw
 Bartolome Sanchez Salado
diff --git a/changelog/426.bugfix.rst b/changelog/426.bugfix.rst
new file mode 100644
index 000000000..caf55dd20
--- /dev/null
+++ b/changelog/426.bugfix.rst
@@ -0,0 +1 @@
+Write directly to stdout buffer if possible to prevent str vs bytes issues - by @asottile
diff --git a/tox/session.py b/tox/session.py
index a414b4fee..8341d9ad9 100644
--- a/tox/session.py
+++ b/tox/session.py
@@ -159,7 +159,12 @@ def popen(self, args, cwd=None, env=None, redirect=True, returnout=False, ignore
             self.report.logpopen(popen, env=env)
             try:
                 if resultjson and not redirect:
-                    assert popen.stderr is None  # prevent deadlock
+                    if popen.stderr is not None:
+                        # prevent deadlock
+                        raise ValueError("stderr must not be piped here")
+                    # we read binary from the process and must write using a
+                    # binary stream
+                    buf = getattr(sys.stdout, 'buffer', sys.stdout)
                     out = None
                     last_time = time.time()
                     while 1:
@@ -167,12 +172,12 @@ def popen(self, args, cwd=None, env=None, redirect=True, returnout=False, ignore
                         # might be no output for a long time with slow tests
                         data = fin.read(1)
                         if data:
-                            sys.stdout.write(data)
+                            buf.write(data)
                             if b'\n' in data or (time.time() - last_time) > 1:
                                 # we flush on newlines or after 1 second to
                                 # provide quick enough feedback to the user
                                 # when printing a dot per test
-                                sys.stdout.flush()
+                                buf.flush()
                                 last_time = time.time()
                         elif popen.poll() is not None:
                             if popen.stdout is not None: