diff --git a/README.md b/README.md
index 16285b0..4213134 100644
--- a/README.md
+++ b/README.md
@@ -51,7 +51,7 @@ axios.get('/user?ID=12345')
   .catch(function (response) {
     console.log(response);
   });
-	
+
 // Optionally the request above could also be done as
 axios.get('/user', {
     params: {
@@ -174,7 +174,7 @@ These are the available config options for making requests. Only the `url` is re
   // The last function in the array must return a string or an ArrayBuffer
   transformRequest: [function (data) {
     // Do whatever you want to transform the data
-	
+
     return data;
   }],
 
@@ -182,7 +182,7 @@ These are the available config options for making requests. Only the `url` is re
   // it is passed to then/catch
   transformResponse: [function (data) {
     // Do whatever you want to transform the data
-	
+
     return data;
   }],
 
@@ -216,6 +216,8 @@ These are the available config options for making requests. Only the `url` is re
   withCredentials: false, // default
 
   // `auth` indicates that HTTP Basic auth should be used, and supplies credentials.
+  // This will set an `Authorization` header, overwriting any existing
+  // `Authorization` custom headers you have set using `headers`.
   // The username can be supplied as `user` or `username`
   // The password can be supplied as `pass` or `password`
   auth: {
@@ -246,7 +248,7 @@ The response for a request contains the following information.
 
   // `status` is the HTTP status code from the server response
   status: 200,
-  
+
   // `statusText` is the HTTP status message from the server response
   statusText: 'OK',
 
diff --git a/lib/adapters/xhr.js b/lib/adapters/xhr.js
index 4fc7b86..61e927f 100644
--- a/lib/adapters/xhr.js
+++ b/lib/adapters/xhr.js
@@ -40,15 +40,15 @@ module.exports = function xhrAdapter(resolve, reject, config) {
   }
 
   // HTTP basic authentication
-  var configAuth = config.auth || {};
-  var auth = {
-    username: configAuth.username || configAuth.user || '',
-    password: configAuth.password || configAuth.pass || ''
-  };
+  if (config.auth) {
+    var username = config.auth.user || config.auth.username || '';
+    var password = config.auth.pass || config.auth.password || '';
+    requestHeaders['Authorization'] = 'Basic: ' + window.btoa(username + ':' + password);
+  }
 
   // Create the request
   var request = new adapter('Microsoft.XMLHTTP');
-  request.open(config.method.toUpperCase(), buildURL(config.url, config.params, config.paramsSerializer), true, auth.username, auth.password);
+  request.open(config.method.toUpperCase(), buildURL(config.url, config.params, config.paramsSerializer), true);
 
   // Set the request timeout in MS
   request.timeout = config.timeout;