diff --git a/src/adapters/adkernel.js b/src/adapters/adkernel.js
index 2b6c4eca83e..32415bcba41 100644
--- a/src/adapters/adkernel.js
+++ b/src/adapters/adkernel.js
@@ -42,7 +42,11 @@ const AdKernelAdapter = function AdKernelAdapter() {
       if (!(zone in _dispatch[host])) {
         _dispatch[host][zone] = [];
       }
-      let imp = {'id': bidId, 'tagid': bid.placementCode, 'banner': {'w': size[0], 'h': size[1]}};
+      let imp = {
+        'id': bidId,
+        'tagid': bid.placementCode,
+        'banner': {'w': size[0], 'h': size[1]}
+      };
       if (utils.getTopWindowLocation().protocol === 'https:') {
         imp.secure = 1;
       }
@@ -55,21 +59,9 @@ const AdKernelAdapter = function AdKernelAdapter() {
       }
       if (syncedHostZones[host].indexOf(zone) === -1) {
         syncedHostZones[host].push(zone);
-        insertUserSync(host, zone);
       }
     };
 
-    function insertUserSync(host, zone) {
-      var iframe = utils.createInvisibleIframe();
-      iframe.src = `//${host}/user-sync?zone=${zone}`;
-      try {
-        document.body.appendChild(iframe);
-      } catch (error) {
-        /* istanbul ignore next */
-        utils.logError(error);
-      }
-    }
-
     /**
      *  Main function to get bid requests
      */
@@ -82,6 +74,16 @@ const AdKernelAdapter = function AdKernelAdapter() {
         });
       });
     };
+    /**
+     *  Build flat user-sync queue from host->zones mapping
+     */
+    this.buildUserSyncQueue = function() {
+      return Object.keys(syncedHostZones)
+        .reduce((m, k) => {
+          syncedHostZones[k].forEach((v) => m.push([k, v]));
+          return m;
+        }, []);
+    };
 
     function dispatchRtbRequest(host, zone, impressions, callback) {
       let url = buildEndpointUrl(host);
@@ -160,7 +162,10 @@ const AdKernelAdapter = function AdKernelAdapter() {
         dispatcher.addImp(bid);
       }
     });
-    // process bids grouped into bidrequests
+    // start async usersync
+    processUserSyncQueue(dispatcher.buildUserSyncQueue());
+
+    // process bids grouped into bid requests
     dispatcher.dispatch((bid, imp, bidResp) => {
       let adUnitId = bid.placementCode;
       if (bidResp) {
@@ -221,6 +226,32 @@ const AdKernelAdapter = function AdKernelAdapter() {
     };
   }
 
+  /**
+   *  Recursively process user-sync queue
+   */
+  function processUserSyncQueue(queue) {
+    if (queue.length === 0) {
+      return;
+    }
+    let entry = queue.pop();
+    insertUserSync(entry[0], entry[1], () => processUserSyncQueue(queue));
+  }
+
+  /**
+   *  Insert single iframe user-sync
+   */
+  function insertUserSync(host, zone, callback) {
+    var iframe = utils.createInvisibleIframe();
+    iframe.src = `//sync.adkernel.com/user-sync?zone=${zone}&r=%2F%2F${host}%2Fuser-synced%3Fuid%3D%7BUID%7D`;
+    utils.addEventHandler(iframe, 'load', callback);
+    try {
+      document.body.appendChild(iframe);
+    } catch (error) {
+      /* istanbul ignore next */
+      utils.logError(error);
+    }
+  }
+
   return {
     callBids: baseAdapter.callBids,
     setBidderCode: baseAdapter.setBidderCode,
diff --git a/test/spec/adapters/adkernel_spec.js b/test/spec/adapters/adkernel_spec.js
index 095a51249fb..dd7eb82060a 100644
--- a/test/spec/adapters/adkernel_spec.js
+++ b/test/spec/adapters/adkernel_spec.js
@@ -238,21 +238,27 @@ describe('Adkernel adapter', () => {
       sandbox.spy(utils, 'createTrackPixelHtml');
       ajaxStub.onCall(0).callsArgWith(1, JSON.stringify(bidResponse1));
       doRequest([bid1_zone1]);
-      expect(bidmanager.addBidResponse.firstCall.args[1].getStatusCode()).to.equal(CONSTANTS.STATUS.GOOD);
       expect(utils.createTrackPixelHtml.calledOnce);
-      let result = pbjs.getBidResponsesForAdUnitCode(bid1_zone1.placementCode);
+      expect(bidmanager.addBidResponse.firstCall.args[1].getStatusCode()).to.equal(CONSTANTS.STATUS.GOOD);
       let expectedNurl = bidResponse1.seatbid[0].bid[0].nurl + '&px=1';
-      expect(result.bids[0].ad).to.include(expectedNurl);
+      expect(bidmanager.addBidResponse.firstCall.args[1].ad).to.include(expectedNurl);
     });
 
     it('should perform usersync for each unique host/zone combination', () => {
       ajaxStub.callsArgWith(1, '');
-      const expectedSyncUrls = ['http://rtb.adkernel.com/user-sync?zone=1', 'http://rtb.adkernel.com/user-sync?zone=2',
-        'http://rtb-private.adkernel.com/user-sync?zone=1'];
-      sandbox.spy(utils, 'createInvisibleIframe');
+      const expectedSyncUrls = ['//sync.adkernel.com/user-sync?zone=1&r=%2F%2Frtb-private.adkernel.com%2Fuser-synced%3Fuid%3D%7BUID%7D',
+        '//sync.adkernel.com/user-sync?zone=2&r=%2F%2Frtb.adkernel.com%2Fuser-synced%3Fuid%3D%7BUID%7D',
+        '//sync.adkernel.com/user-sync?zone=1&r=%2F%2Frtb.adkernel.com%2Fuser-synced%3Fuid%3D%7BUID%7D'];
+      let userSyncUrls = [];
+      sandbox.stub(utils, 'createInvisibleIframe', () => {
+        return {};
+      });
+      sandbox.stub(utils, 'addEventHandler', (el, ev, cb) => {
+        userSyncUrls.push(el.src);
+        cb(); // instant callback
+      });
       doRequest([bid1_zone1, bid2_zone2, bid2_zone2, bid3_host2]);
       expect(utils.createInvisibleIframe.calledThrice);
-      let userSyncUrls = utils.createInvisibleIframe.returnValues.map(val => val.src);
       expect(userSyncUrls).to.be.eql(expectedSyncUrls);
     });
   });