diff --git a/lib/transport.js b/lib/transport.js
index 03104de3..144ba730 100644
--- a/lib/transport.js
+++ b/lib/transport.js
@@ -35,6 +35,7 @@ export class Basic extends Transport {
   send(request, url, options) {
     return co(function *() {
       let sandbox = options && options.sandbox;
+      let asynchronous = options && options.async !== undefined ? options.async : true;
       let transformRequest = request.transformRequest;
       let transformResponse = request.transformResponse;
       let onerror = request.onerror;
@@ -44,7 +45,7 @@ export class Basic extends Transport {
       xhr.open(
         request.method,
         url,
-        true /* async */,
+        asynchronous,
         this.credentials.username,
         this.credentials.password
       );
@@ -76,6 +77,7 @@ export class OAuth2 extends Transport {
   send(request, url, options={}) {
     return co(function *() {
       let sandbox = options.sandbox;
+      let asynchronous = options && options.async !== undefined ? options.async : true;
       let transformRequest = request.transformRequest;
       let transformResponse = request.transformResponse;
       let onerror = request.onerror;
@@ -87,7 +89,7 @@ export class OAuth2 extends Transport {
         let token = yield access(this.credentials, options);
         xhr = new XMLHttpRequest();
         if (sandbox) sandbox.add(xhr);
-        xhr.open(request.method, url, true /* async */);
+        xhr.open(request.method, url, asynchronous);
         xhr.setRequestHeader('Authorization', `Bearer ${token}`);
         if (transformRequest) transformRequest(xhr);
         yield xhr.send(request.requestData);
@@ -132,9 +134,10 @@ function isExpired(credentials) {
 
 let getAccessToken = co.wrap(function *(credentials, options) {
   let sandbox = options.sandbox;
+  let asynchronous = options && options.async !== undefined ? options.async : true;
   let xhr = new XMLHttpRequest();
   if (sandbox) sandbox.add(xhr);
-  xhr.open('POST', credentials.tokenUrl, true /* async */);
+  xhr.open('POST', credentials.tokenUrl, asynchronous);
   xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
 
   let data = querystring.stringify({