diff --git a/src/Workflow/Mutex.php b/src/Workflow/Mutex.php
index 639d4b6c..0b20f857 100644
--- a/src/Workflow/Mutex.php
+++ b/src/Workflow/Mutex.php
@@ -34,7 +34,7 @@ final class Mutex
      *  yield $this->mutex->lock();
      * ```
      *
-     * @return PromiseInterface A promise that resolves when the lock is acquired.
+     * @return PromiseInterface<self> A promise that resolves when the lock is acquired.
      */
     public function lock(): PromiseInterface
     {
diff --git a/tests/Unit/Workflow/MutexTestCase.php b/tests/Unit/Workflow/MutexTestCase.php
new file mode 100644
index 00000000..dbd1428f
--- /dev/null
+++ b/tests/Unit/Workflow/MutexTestCase.php
@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Temporal\Tests\Unit\Workflow;
+
+use PHPUnit\Framework\TestCase;
+use Temporal\Workflow\Mutex;
+
+final class MutexTestCase extends TestCase
+{
+    public function testIsLockedLockUnlock(): void
+    {
+        $mutex = new Mutex();
+
+        $this->assertFalse($mutex->isLocked());
+        $mutex->lock();
+        $this->assertTrue($mutex->isLocked());
+        $mutex->unlock();
+        $this->assertFalse($mutex->isLocked());
+    }
+
+    public function testTryLock(): void
+    {
+        $mutex = new Mutex();
+
+        $this->assertTrue($mutex->tryLock());
+        $this->assertFalse($mutex->tryLock());
+        $mutex->unlock();
+        $this->assertTrue($mutex->tryLock());
+    }
+
+    public function testLock(): void
+    {
+        $result = [false, false, false];
+
+        $mutex = new Mutex();
+        $this->assertTrue($mutex->tryLock());
+
+        $mutex->lock()->then(function (Mutex $mutex) use (&$result) {
+            $result[0] = true;
+            $mutex->unlock();
+        });
+        $mutex->lock()->then(function () use (&$result) {
+            $result[1] = true;
+        });
+        $mutex->lock()->then(function () use (&$result) {
+            $result[2] = true;
+        });
+
+
+        $this->assertSame([false, false, false], $result);
+
+        $mutex->unlock();
+        $this->assertSame([true, true, false], $result);
+
+        $mutex->unlock();
+        $this->assertSame([true, true, true], $result);
+    }
+}