diff --git a/webrtc/modules/audio_device/ios/audio_device_ios.mm b/webrtc/modules/audio_device/ios/audio_device_ios.mm
index e094ebdc3d..b0d26be6ec 100644
--- a/webrtc/modules/audio_device/ios/audio_device_ios.mm
+++ b/webrtc/modules/audio_device/ios/audio_device_ios.mm
@@ -86,6 +86,12 @@ const UInt32 kBytesPerSample = 2;
 // Can most likely be removed.
 const UInt16 kFixedPlayoutDelayEstimate = 30;
 const UInt16 kFixedRecordDelayEstimate = 30;
+// Calls to AudioUnitInitialize() can fail if called back-to-back on different
+// ADM instances. A fall-back solution is to allow multiple sequential calls
+// with as small delay between each. This factor sets the max number of allowed
+// initialization attempts.
+const int kMaxNumberOfAudioUnitInitializeAttempts = 5;
+
 
 using ios::CheckAndLogError;
 
@@ -741,6 +747,7 @@ bool AudioDeviceIOS::SetupAndInitializeVoiceProcessingAudioUnit() {
   vpio_unit_description.componentManufacturer = kAudioUnitManufacturer_Apple;
   vpio_unit_description.componentFlags = 0;
   vpio_unit_description.componentFlagsMask = 0;
+
   // Obtain an audio unit instance given the description.
   AudioComponent found_vpio_unit_ref =
       AudioComponentFindNext(nullptr, &vpio_unit_description);
@@ -876,17 +883,28 @@ bool AudioDeviceIOS::SetupAndInitializeVoiceProcessingAudioUnit() {
   }
 
   // Initialize the Voice-Processing I/O unit instance.
+  // Calls to AudioUnitInitialize() can fail if called back-to-back on
+  // different ADM instances. The error message in this case is -66635 which is
+  // undocumented. Tests have shown that calling AudioUnitInitialize a second
+  // time, after a short sleep, avoids this issue.
+  // See webrtc:5166 for details.
+  int failed_initalize_attempts = 0;
   result = AudioUnitInitialize(vpio_unit_);
-  if (result != noErr) {
-    result = AudioUnitUninitialize(vpio_unit_);
-    if (result != noErr) {
-      LOG_F(LS_ERROR) << "AudioUnitUninitialize failed: " << result;
-    }
-    DisposeAudioUnit();
+  while (result != noErr) {
     LOG(LS_ERROR) << "Failed to initialize the Voice-Processing I/O unit: "
                   << result;
-    return false;
+    ++failed_initalize_attempts;
+    if (failed_initalize_attempts == kMaxNumberOfAudioUnitInitializeAttempts) {
+      // Max number of initialization attempts exceeded, hence abort.
+      LOG(LS_WARNING) << "Too many initialization attempts";
+      DisposeAudioUnit();
+      return false;
+    }
+    LOG(LS_INFO) << "pause 100ms and try audio unit initialization again...";
+    [NSThread sleepForTimeInterval:0.1f];
+    result = AudioUnitInitialize(vpio_unit_);
   }
+  LOG(LS_INFO) << "Voice-Processing I/O unit is now initialized";
   return true;
 }
 
diff --git a/webrtc/modules/audio_device/ios/audio_device_unittest_ios.cc b/webrtc/modules/audio_device/ios/audio_device_unittest_ios.cc
index b892f28b1d..8993aceea4 100644
--- a/webrtc/modules/audio_device/ios/audio_device_unittest_ios.cc
+++ b/webrtc/modules/audio_device/ios/audio_device_unittest_ios.cc
@@ -636,6 +636,55 @@ TEST_F(AudioDeviceTest, StopPlayoutRequiresInitToRestart) {
   EXPECT_FALSE(audio_device()->PlayoutIsInitialized());
 }
 
+// Verify that we can create two ADMs and start playing on the second ADM.
+// Only the first active instance shall activate an audio session and the
+// last active instace shall deactivate the audio session.
+TEST_F(AudioDeviceTest, StartPlayoutOnTwoInstances) {
+  // Create and initialize a second/extra ADM instance. The default ADM is
+  // created by the test harness.
+  rtc::scoped_refptr<AudioDeviceModule> second_audio_device =
+      CreateAudioDevice(AudioDeviceModule::kPlatformDefaultAudio);
+  EXPECT_NE(second_audio_device.get(), nullptr);
+  EXPECT_EQ(0, second_audio_device->Init());
+
+  // Start playout for the default ADM. Ignore the callback sequence.
+  NiceMock<MockAudioTransport> mock(kPlayout);
+  EXPECT_EQ(0, audio_device()->RegisterAudioCallback(&mock));
+  StartPlayout();
+
+  // Initialize playout for the second ADM. If all is OK, the second ADM shall
+  // reuse the audio session activated when the first ADM started playing.
+  // This call will also ensure that we avoid a problem related to initializing
+  // two different audio unit instances back to back (see webrtc:5166 for
+  // details).
+  EXPECT_EQ(0, second_audio_device->InitPlayout());
+  EXPECT_TRUE(second_audio_device->PlayoutIsInitialized());
+
+  // Stop playout for the default ADM. The audio session shall not be
+  // deactivated since it is used by the second ADM.
+  StopPlayout();
+
+  // Start playout for the second ADM and verify that it starts as intended.
+  // Passing this test ensures that initialization of the second audio unit
+  // has been done successfully.
+  MockAudioTransport mock2(kPlayout);
+  mock2.HandleCallbacks(test_is_done_.get(), nullptr, kNumCallbacks);
+  EXPECT_CALL(
+      mock2, NeedMorePlayData(playout_frames_per_10ms_buffer(), kBytesPerSample,
+                              playout_channels(), playout_sample_rate(),
+                              NotNull(), _, _, _))
+      .Times(AtLeast(kNumCallbacks));
+  EXPECT_EQ(0, second_audio_device->RegisterAudioCallback(&mock2));
+  EXPECT_EQ(0, second_audio_device->StartPlayout());
+  EXPECT_TRUE(second_audio_device->Playing());
+  test_is_done_->Wait(kTestTimeOutInMilliseconds);
+  EXPECT_EQ(0, second_audio_device->StopPlayout());
+  EXPECT_FALSE(second_audio_device->Playing());
+  EXPECT_FALSE(second_audio_device->PlayoutIsInitialized());
+
+  EXPECT_EQ(0, second_audio_device->Terminate());
+}
+
 // Start playout and verify that the native audio layer starts asking for real
 // audio samples to play out using the NeedMorePlayData callback.
 TEST_F(AudioDeviceTest, StartPlayoutVerifyCallbacks) {