diff --git a/lib/cpp/src/concurrency/ThreadManager.cpp b/lib/cpp/src/concurrency/ThreadManager.cpp
index 7bba0e6..52473c7 100644
--- a/lib/cpp/src/concurrency/ThreadManager.cpp
+++ b/lib/cpp/src/concurrency/ThreadManager.cpp
@@ -54,7 +54,9 @@
     workerMaxCount_(0),
     idleCount_(0),
     pendingTaskCountMax_(0),
-    state_(ThreadManager::UNINITIALIZED) {}
+    state_(ThreadManager::UNINITIALIZED),
+    monitor_(&mutex_),
+    maxMonitor_(&mutex_) {}
 
   ~Impl() { stop(); }
 
@@ -133,7 +135,9 @@
 
   friend class ThreadManager::Task;
   std::queue<shared_ptr<Task> > tasks_;
+  Mutex mutex_;
   Monitor monitor_;
+  Monitor maxMonitor_;
   Monitor workerMonitor_;
 
   friend class ThreadManager::Worker;
@@ -245,7 +249,7 @@
        * the manager will see it.
        */
       {
-        Synchronized s(manager_->monitor_);
+        Guard g(manager_->mutex_);
         active = isActive();
 
         while (active && manager_->tasks_.empty()) {
@@ -269,7 +273,7 @@
                thread that might be blocked on add. */
             if (manager_->pendingTaskCountMax_ != 0 &&
                 manager_->tasks_.size() == manager_->pendingTaskCountMax_ - 1) {
-              manager_->monitor_.notify();
+              manager_->maxMonitor_.notify();
             }
           }
         } else {
@@ -432,7 +436,7 @@
   }
 
   void ThreadManager::Impl::add(shared_ptr<Runnable> value, int64_t timeout) {
-    Synchronized s(monitor_);
+    Guard g(mutex_);
 
     if (state_ != ThreadManager::STARTED) {
       throw IllegalStateException();
@@ -441,7 +445,8 @@
     if (pendingTaskCountMax_ > 0 && (tasks_.size() >= pendingTaskCountMax_)) {
       if (canSleep() && timeout >= 0) {
         while (pendingTaskCountMax_ > 0 && tasks_.size() >= pendingTaskCountMax_) {
-          monitor_.wait(timeout);
+          // This is thread safe because the mutex is shared between monitors.
+          maxMonitor_.wait(timeout);
         }
       } else {
         throw TooManyPendingTasksException();