diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index 2f0fba9eb2..5ad55988e9 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -1388,6 +1388,7 @@ if (rtc_include_tests) {
       "../test:test_main",
       "../test:test_support",
       "memory:fifo_buffer",
+      "synchronization:mutex",
       "synchronization:synchronization_unittests",
       "task_utils:to_queued_task",
       "third_party/sigslot",
diff --git a/rtc_base/deprecated/signal_thread_unittest.cc b/rtc_base/deprecated/signal_thread_unittest.cc
index c280e970be..f5a49aad63 100644
--- a/rtc_base/deprecated/signal_thread_unittest.cc
+++ b/rtc_base/deprecated/signal_thread_unittest.cc
@@ -13,9 +13,9 @@
 #include <memory>
 
 #include "rtc_base/constructor_magic.h"
-#include "rtc_base/critical_section.h"
 #include "rtc_base/gunit.h"
 #include "rtc_base/null_socket_server.h"
+#include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/thread.h"
 #include "rtc_base/thread_annotations.h"
 #include "test/gtest.h"
@@ -148,13 +148,13 @@ class OwnerThread : public Thread, public sigslot::has_slots<> {
     // Delete |signal_thread|.
     signal_thread->Destroy(true);
     {
-      rtc::CritScope cs(&crit_);
+      webrtc::MutexLock lock(&mutex_);
       has_run_ = true;
     }
   }
 
   bool has_run() {
-    rtc::CritScope cs(&crit_);
+    webrtc::MutexLock lock(&mutex_);
     return has_run_;
   }
   void OnWorkDone(DEPRECATED_SignalThread* /*signal_thread*/) {
@@ -162,9 +162,9 @@ class OwnerThread : public Thread, public sigslot::has_slots<> {
   }
 
  private:
-  rtc::CriticalSection crit_;
+  webrtc::Mutex mutex_;
   SignalThreadTest* harness_;
-  bool has_run_ RTC_GUARDED_BY(crit_);
+  bool has_run_ RTC_GUARDED_BY(mutex_);
   RTC_DISALLOW_COPY_AND_ASSIGN(OwnerThread);
 };
 
diff --git a/rtc_base/thread_unittest.cc b/rtc_base/thread_unittest.cc
index bc0c3442eb..d3cae34dfa 100644
--- a/rtc_base/thread_unittest.cc
+++ b/rtc_base/thread_unittest.cc
@@ -22,12 +22,14 @@
 #include "rtc_base/null_socket_server.h"
 #include "rtc_base/physical_socket_server.h"
 #include "rtc_base/socket_address.h"
+#include "rtc_base/synchronization/mutex.h"
 #include "rtc_base/task_utils/to_queued_task.h"
 #include "rtc_base/third_party/sigslot/sigslot.h"
 #include "test/testsupport/rtc_expect_death.h"
 
 #if defined(WEBRTC_WIN)
 #include <comdef.h>  // NOLINT
+
 #endif
 
 namespace rtc {
@@ -161,17 +163,17 @@ class AtomicBool {
  public:
   explicit AtomicBool(bool value = false) : flag_(value) {}
   AtomicBool& operator=(bool value) {
-    CritScope scoped_lock(&cs_);
+    webrtc::MutexLock scoped_lock(&mutex_);
     flag_ = value;
     return *this;
   }
   bool get() const {
-    CritScope scoped_lock(&cs_);
+    webrtc::MutexLock scoped_lock(&mutex_);
     return flag_;
   }
 
  private:
-  CriticalSection cs_;
+  mutable webrtc::Mutex mutex_;
   bool flag_;
 };
 
@@ -413,18 +415,18 @@ TEST(ThreadTest, ThreeThreadsInvoke) {
     explicit LockedBool(bool value) : value_(value) {}
 
     void Set(bool value) {
-      CritScope lock(&crit_);
+      webrtc::MutexLock lock(&mutex_);
       value_ = value;
     }
 
     bool Get() {
-      CritScope lock(&crit_);
+      webrtc::MutexLock lock(&mutex_);
       return value_;
     }
 
    private:
-    CriticalSection crit_;
-    bool value_ RTC_GUARDED_BY(crit_);
+    webrtc::Mutex mutex_;
+    bool value_ RTC_GUARDED_BY(mutex_);
   };
 
   struct LocalFuncs {
@@ -447,7 +449,6 @@ TEST(ThreadTest, ThreeThreadsInvoke) {
                                       Thread* thread1,
                                       Thread* thread2,
                                       LockedBool* out) {
-      CriticalSection crit;
       LockedBool async_invoked(false);
 
       invoker->AsyncInvoke<void>(
diff --git a/test/network/BUILD.gn b/test/network/BUILD.gn
index 949b201c72..35673741ce 100644
--- a/test/network/BUILD.gn
+++ b/test/network/BUILD.gn
@@ -75,6 +75,7 @@ rtc_library("network_emulation_unittest") {
     "../../rtc_base:gunit_helpers",
     "../../rtc_base:logging",
     "../../rtc_base:rtc_event",
+    "../../rtc_base/synchronization:mutex",
     "../../system_wrappers:system_wrappers",
   ]
 }
diff --git a/test/network/network_emulation_unittest.cc b/test/network/network_emulation_unittest.cc
index 9e630de9cb..6bd3b6ebc7 100644
--- a/test/network/network_emulation_unittest.cc
+++ b/test/network/network_emulation_unittest.cc
@@ -19,6 +19,7 @@
 #include "call/simulated_network.h"
 #include "rtc_base/event.h"
 #include "rtc_base/gunit.h"
+#include "rtc_base/synchronization/mutex.h"
 #include "system_wrappers/include/sleep.h"
 #include "test/gmock.h"
 #include "test/gtest.h"
@@ -48,12 +49,12 @@ class SocketReader : public sigslot::has_slots<> {
     int64_t timestamp;
     len_ = socket_->Recv(buf_, size_, &timestamp);
 
-    rtc::CritScope crit(&lock_);
+    MutexLock lock(&lock_);
     received_count_++;
   }
 
   int ReceivedCount() {
-    rtc::CritScope crit(&lock_);
+    MutexLock lock(&lock_);
     return received_count_;
   }
 
@@ -64,7 +65,7 @@ class SocketReader : public sigslot::has_slots<> {
   size_t size_;
   int len_;
 
-  rtc::CriticalSection lock_;
+  Mutex lock_;
   int received_count_ RTC_GUARDED_BY(lock_) = 0;
 };