diff -r 070cc3b9d5cc Lib/test/test_os.py
--- a/Lib/test/test_os.py	Wed Sep 14 15:04:33 2016 +0200
+++ b/Lib/test/test_os.py	Wed Sep 14 14:13:15 2016 +0000
@@ -2165,8 +2165,8 @@
 
 
 class SpawnTests(unittest.TestCase):
-    def create_args(self, with_env=False):
-        self.exitcode = 17
+    def create_args(self, with_env=False, use_bytes=False):
+        self.exitcode = 17        
 
         filename = support.TESTFN
         self.addCleanup(support.unlink, filename)
@@ -2185,7 +2185,13 @@
         with open(filename, "w") as fp:
             fp.write(code)
 
-        return [sys.executable, filename]
+        args = [sys.executable, filename]
+        if use_bytes:
+            args = [os.fsencode(a) for a in args]
+            self.env = {os.fsencode(k):os.fsencode(v)
+                           for k, v in self.env.items()}
+
+        return args
 
     @unittest.skipUnless(hasattr(os, 'spawnl'), 'need os.spawnl')
     def test_spawnl(self):
@@ -2248,6 +2254,13 @@
         else:
             self.assertEqual(status, self.exitcode << 8)
 
+    @unittest.skipUnless(hasattr(os, 'spawnve'), 'need os.spawnve')
+    def test_spawnve_bytes(self):
+        # Test bytes handling in parse_arglist and parse_envlist.
+        # See issue 28114.
+        args = self.create_args(True, use_bytes=True)
+        exitcode = os.spawnve(os.P_WAIT, args[0], args, self.env)
+        self.assertEqual(exitcode, self.exitcode)
 
 # The introduction of this TestCase caused at least two different errors on
 # *nix buildbots. Temporarily skip this to let the buildbots move along.
diff -r 070cc3b9d5cc Modules/posixmodule.c
--- a/Modules/posixmodule.c	Wed Sep 14 15:04:33 2016 +0200
+++ b/Modules/posixmodule.c	Wed Sep 14 14:13:15 2016 +0000
@@ -4729,28 +4729,31 @@
     PyMem_DEL(array);
 }
 
-static
-int fsconvert_strdup(PyObject *o, EXECV_CHAR**out)
+static int
+fsconvert_strdup(PyObject *o, EXECV_CHAR**out)
 {
     Py_ssize_t size;
+    PyObject *ub;
+    int result = 0;
 #if defined(HAVE_WEXECV) || defined(HAVE_WSPAWNV)
-    *out = PyUnicode_AsWideCharString(o, &size);
-    if (!*out)
+    if (!PyUnicode_FSDecoder(o, &ub))
         return 0;
-#else
-    PyObject *bytes;
-    if (!PyUnicode_FSConverter(o, &bytes))
+    *out = PyUnicode_AsWideCharString(ub, &size);
+    if (*out)
+        result = 1;
+#else
+    if (!PyUnicode_FSConverter(o, &ub))
         return 0;
-    size = PyBytes_GET_SIZE(bytes);
-    *out = PyMem_Malloc(size+1);
-    if (!*out) {
+    size = PyBytes_GET_SIZE(ub);
+    *out = PyMem_Malloc(size + 1);
+    if (*out) {
+        memcpy(*out, PyBytes_AS_STRING(ub), size + 1);
+        result = 1;
+    } else
         PyErr_NoMemory();
-        return 0;
-    }
-    memcpy(*out, PyBytes_AsString(bytes), size+1);
-    Py_DECREF(bytes);
-#endif
-    return 1;
+#endif
+    Py_DECREF(ub);
+    return result;
 }
 #endif
 
@@ -4760,7 +4763,7 @@
 {
     Py_ssize_t i, pos, envc;
     PyObject *keys=NULL, *vals=NULL;
-    PyObject *key, *val, *keyval;
+    PyObject *key, *val, *key2, *val2, *keyval;
     EXECV_CHAR **envlist;
 
     i = PyMapping_Size(env);
@@ -4790,15 +4793,32 @@
         if (!key || !val)
             goto error;
 
-        keyval = PyUnicode_FromFormat("%U=%U", key, val);
+#if defined(HAVE_WEXECV) || defined(HAVE_WSPAWNV)
+        if (!PyUnicode_FSDecoder(key, &key2))
+            goto error;
+        if (!PyUnicode_FSDecoder(val, &val2)) {
+            Py_DECREF(key2);
+            goto error;
+        }
+        keyval = PyUnicode_FromFormat("%U=%U", key2, val2);
+#else
+        if (!PyUnicode_FSConverter(key, &key2))
+            goto error;
+        if (!PyUnicode_FSConverter(val, &val2)) {
+            Py_DECREF(key2);
+            goto error;
+        }
+        keyval = PyBytes_FromFormat("%s=%s", PyBytes_AS_STRING(key2),
+                                             PyBytes_AS_STRING(val2));
+#endif
+        Py_DECREF(key2);
+        Py_DECREF(val2);
         if (!keyval)
             goto error;
-
         if (!fsconvert_strdup(keyval, &envlist[envc++])) {
             Py_DECREF(keyval);
             goto error;
         }
-        
         Py_DECREF(keyval);
     }
     Py_DECREF(vals);