diff --git a/trac/tests/functional/testcases.py b/trac/tests/functional/testcases.py
index 4c8414d844..2a34919042 100755
--- a/trac/tests/functional/testcases.py
+++ b/trac/tests/functional/testcases.py
@@ -304,6 +304,7 @@ def runTest(self):
             # are used and the req.perm has no permissions.
             tc.notfind(internal_error)
             tc.notfind("You don't have the required permissions")
+            tc.find('>logged in as <span class="trac-author-user">joé</span>')
             self._tester.logout()
             # finally restore expected 'admin' login
             self._tester.login('admin')
diff --git a/trac/web/api.py b/trac/web/api.py
index c944b005c7..80cf2ede85 100644
--- a/trac/web/api.py
+++ b/trac/web/api.py
@@ -470,6 +470,33 @@ def arg_list_to_args(arg_list):
     return args
 
 
+if hasattr(str, 'isascii'):
+    _isascii = lambda value: value.isascii()
+else:
+    _is_non_ascii_re = re.compile(r'[^\x00-\x7f]')
+    _isascii = lambda value: not _is_non_ascii_re.search(value)
+
+
+def wsgi_string_decode(value):
+    """Convert from a WSGI "bytes-as-unicode" string to an unicode string.
+    """
+    if not isinstance(value, str):
+        raise TypeError('Must a str instance rather than %s' % type(value))
+    if not _isascii(value):
+        value = value.encode('iso-8859-1').decode('utf-8')
+    return value
+
+
+def wsgi_string_encode(value):
+    """Convert from an unicode string to a WSGI "bytes-as-unicode" string.
+    """
+    if not isinstance(value, str):
+        raise TypeError('Must a str instance rather than %s' % type(value))
+    if not _isascii(value):
+        value = value.encode('utf-8').decode('iso-8859-1')
+    return value
+
+
 def _raise_if_null_bytes(value):
     if value and '\x00' in value:
         raise HTTPBadRequest(_("Invalid request arguments."))
@@ -666,7 +693,7 @@ def __getattr__(self, name):
 
     def __repr__(self):
         uri = self.environ.get('PATH_INFO', '')
-        qs = self.query_string
+        qs = self.environ.get('QUERY_STRING', '')
         if qs:
             uri += '?' + qs
         return '<%s "%s %r">' % (self.__class__.__name__, self.method, uri)
@@ -698,21 +725,16 @@ def method(self):
     def path_info(self):
         """Path inside the application"""
         path_info = self.environ.get('PATH_INFO', '')
-        if isinstance(path_info, str):
-            # According to PEP 3333, the value is decoded by iso-8859-1
-            # encoding when it is a unicode string. However, we need
-            # decoded unicode string by utf-8 encoding.
-            path_info = path_info.encode('iso-8859-1')
         try:
-            return str(path_info, 'utf-8')
-        except UnicodeDecodeError:
+            return wsgi_string_decode(path_info)
+        except UnicodeError:
             raise HTTPNotFound(_("Invalid URL encoding (was %(path_info)r)",
                                  path_info=path_info))
 
     @property
     def query_string(self):
         """Query part of the request"""
-        return self.environ.get('QUERY_STRING', '')
+        return wsgi_string_decode(self.environ.get('QUERY_STRING', ''))
 
     @property
     def remote_addr(self):
@@ -727,7 +749,7 @@ def remote_user(self):
         """
         user = self.environ.get('REMOTE_USER')
         if user is not None:
-            return to_unicode(user)
+            return wsgi_string_decode(user)
 
     @property
     def request_path(self):
@@ -745,7 +767,7 @@ def scheme(self):
     @property
     def base_path(self):
         """The root path of the application"""
-        return self.environ.get('SCRIPT_NAME', '')
+        return wsgi_string_decode(self.environ.get('SCRIPT_NAME', ''))
 
     @property
     def server_name(self):
diff --git a/trac/web/main.py b/trac/web/main.py
index dc5bccb8d7..b8c6b9cc62 100644
--- a/trac/web/main.py
+++ b/trac/web/main.py
@@ -53,7 +53,8 @@
                          HTTPInternalServerError, HTTPNotFound, IAuthenticator, \
                          IRequestFilter, IRequestHandler, Request, \
                          RequestDone, TracNotImplementedError, \
-                         is_valid_default_handler, parse_header
+                         is_valid_default_handler, parse_header, \
+                         wsgi_string_decode, wsgi_string_encode
 from trac.web.chrome import Chrome, ITemplateProvider, add_notice, \
                             add_stylesheet, add_warning
 from trac.web.href import Href
@@ -535,14 +536,14 @@ def dispatch_request(environ, start_response):
             # the remaining path in the `PATH_INFO` variable.
             script_name = environ.get('SCRIPT_NAME', '')
             try:
-                if isinstance(script_name, str):
-                    script_name = script_name.encode('iso-8859-1')  # PEP 3333
-                script_name = str(script_name, 'utf-8')
+                script_name = wsgi_string_decode(script_name)
+                env_name = wsgi_string_decode(env_name)
             except UnicodeDecodeError:
                 errmsg = 'Invalid URL encoding (was %r)' % script_name
             else:
                 # (as Href expects unicode parameters)
-                environ['SCRIPT_NAME'] = Href(script_name)(env_name)
+                script_name = wsgi_string_encode(Href(script_name)(env_name))
+                environ['SCRIPT_NAME'] = script_name
                 environ['PATH_INFO'] = '/' + '/'.join(path_info)
 
                 if env_parent_dir:
diff --git a/trac/web/standalone.py b/trac/web/standalone.py
index 5929dc038f..44775e2f71 100755
--- a/trac/web/standalone.py
+++ b/trac/web/standalone.py
@@ -32,6 +32,7 @@
 from trac import __version__ as VERSION
 from trac.util import autoreload, daemon
 from trac.util.text import printerr
+from trac.web.api import wsgi_string_encode
 from trac.web.auth import BasicAuthentication, DigestAuthentication
 from trac.web.main import dispatch_request
 from trac.web.wsgi import WSGIServer, WSGIRequestHandler
@@ -59,7 +60,7 @@ def __call__(self, environ, start_response):
                     remote_user = auth.do_auth(environ, start_response)
                     if not remote_user:
                         return []
-                    environ['REMOTE_USER'] = remote_user
+                    environ['REMOTE_USER'] = wsgi_string_encode(remote_user)
         return self.application(environ, start_response)
 
 
diff --git a/trac/web/tests/api.py b/trac/web/tests/api.py
index 8ce5a002b7..a43e1a9a7e 100644
--- a/trac/web/tests/api.py
+++ b/trac/web/tests/api.py
@@ -702,6 +702,48 @@ def test_check_modified_if_none_match(self):
             req.send(b'')
         self.assertEqual(etag, req.headers_sent['ETag'])
 
+    def test_path_info(self):
+
+        def test(expected, value):
+            environ = _make_environ(PATH_INFO=value)
+            self.assertEqual(expected, _make_req(environ).path_info)
+
+        test('', '')
+        test('/wiki/WikiStart', '/wiki/WikiStart')
+        test('/wiki/TæstPäge', '/wiki/T\xc3\xa6stP\xc3\xa4ge')
+
+    def test_query_string(self):
+
+        def test(expected, value):
+            environ = _make_environ(QUERY_STRING=value)
+            self.assertEqual(expected, _make_req(environ).query_string)
+
+        test('', '')
+        test('status=defect&milestone=milestone1',
+             'status=defect&milestone=milestone1')
+        test('status=defećt&milestóne=milestone1',
+             'status=defe\xc4\x87t&milest\xc3\xb3ne=milestone1')
+
+    def test_base_path(self):
+
+        def test(expected, value):
+            environ = _make_environ(SCRIPT_NAME=value)
+            self.assertEqual(expected, _make_req(environ).base_path)
+
+        test('', '')
+        test('/1.6-stable', '/1.6-stable')
+        test('/Prøjeçt-42', '/Pr\xc3\xb8je\xc3\xa7t-42')
+
+    def test_remote_user(self):
+
+        def test(expected, value):
+            environ = _make_environ(REMOTE_USER=value)
+            self.assertEqual(expected, _make_req(environ).remote_user)
+
+        test('', '')
+        test('joe', 'joe')
+        test('jöhn', 'j\xc3\xb6hn')
+
 
 class RequestSendFileTestCase(unittest.TestCase):