diff --git a/httpx/_client.py b/httpx/_client.py
index 645c83e0f1..fff3668058 100644
--- a/httpx/_client.py
+++ b/httpx/_client.py
@@ -71,11 +71,11 @@ def __init__(
     ):
         self._base_url = self._enforce_trailing_slash(URL(base_url))
 
-        self.auth = auth
+        self._auth = auth
         self._params = QueryParams(params)
         self._headers = Headers(headers)
         self._cookies = Cookies(cookies)
-        self.timeout = Timeout(timeout)
+        self._timeout = Timeout(timeout)
         self.max_redirects = max_redirects
         self._trust_env = trust_env
         self._netrc = NetRCInfo()
@@ -109,6 +109,22 @@ def _get_proxy_map(
             proxy = Proxy(url=proxies) if isinstance(proxies, (str, URL)) else proxies
             return {"all": proxy}
 
+    @property
+    def auth(self) -> typing.Optional[AuthTypes]:
+        return self._auth
+
+    @auth.setter
+    def auth(self, auth: typing.Optional[AuthTypes]) -> None:
+        self._auth = auth
+
+    @property
+    def timeout(self) -> Timeout:
+        return self._timeout
+
+    @timeout.setter
+    def timeout(self, timeout: Timeout) -> None:
+        self._timeout = Timeout(timeout)
+
     @property
     def base_url(self) -> URL:
         """