diff --git a/library/grow/pump.py b/library/grow/pump.py
index 9a21d9e..6c73f19 100644
--- a/library/grow/pump.py
+++ b/library/grow/pump.py
@@ -10,6 +10,9 @@ PUMP_PWM_FREQ = 10000
 PUMP_MAX_DUTY = 90
 
 
+global_lock = threading.Lock()
+
+
 class Pump(object):
     """Grow pump driver."""
 
@@ -42,8 +45,11 @@ class Pump(object):
         """Set pump speed (PWM duty cycle)."""
         if speed > 1.0 or speed < 0:
             raise ValueError("Speed must be between 0 and 1")
+        if not global_lock.acquire(blocking=False):
+            return False
         self._pwm.ChangeDutyCycle(int(PUMP_MAX_DUTY * speed))
         self._speed = speed
+        return True
 
     def get_speed(self):
         """Return Pump speed (PWM duty cycle)."""
@@ -55,6 +61,7 @@ class Pump(object):
         if self._timeout is not None:
             self._timeout.cancel()
             self._timeout = None
+        global_lock.release()
 
     def dose(self, speed, timeout=0.1, blocking=True, force=False):
         """Pulse the pump for timeout seconds.
@@ -64,19 +71,22 @@ class Pump(object):
         :param force: Applies only to non-blocking. If true, any previous dose will be replaced
 
         """
+
         if blocking:
-            self.set_speed(speed)
-            time.sleep(timeout)
-            self.stop()
-            return True
+            if self.set_speed(speed):
+                time.sleep(timeout)
+                self.stop()
+                return True
+
         else:
             if self._timeout is not None:
                 if self._timeout.is_alive():
                     if force:
                         self._timeout.cancel()
-                    else:
-                        return False
+
             self._timeout = threading.Timer(timeout, self.stop)
-            self.set_speed(speed)
-            self._timeout.start()
-            return True
+            if self.set_speed(speed):
+                self._timeout.start()
+                return True
+
+        return False
diff --git a/library/tests/test_lock.py b/library/tests/test_lock.py
new file mode 100644
index 0000000..8901bd6
--- /dev/null
+++ b/library/tests/test_lock.py
@@ -0,0 +1,37 @@
+import time
+
+
+def test_pumps_are_mutually_exclusive(GPIO, smbus):
+    from grow.pump import Pump, global_lock
+
+    ch1 = Pump(channel=1)
+    ch2 = Pump(channel=2)
+    ch3 = Pump(channel=3)
+
+    ch1.dose(speed=0.5, timeout=1.0, blocking=False)
+
+    assert global_lock.locked() is True
+
+    assert ch2.dose(speed=0.5) is False
+    assert ch2.dose(speed=0.5, blocking=False) is False
+
+    assert ch3.dose(speed=0.5) is False
+    assert ch3.dose(speed=0.5, blocking=False) is False
+
+
+def test_pumps_run_sequentially(GPIO, smbus):
+    from grow.pump import Pump, global_lock
+
+    ch1 = Pump(channel=1)
+    ch2 = Pump(channel=2)
+    ch3 = Pump(channel=3)
+
+    assert ch1.dose(speed=0.5, timeout=0.1, blocking=False) is True
+    assert global_lock.locked() is True
+    time.sleep(0.3)
+    assert ch2.dose(speed=0.5, timeout=0.1, blocking=False) is True
+    assert global_lock.locked() is True
+    time.sleep(0.3)
+    assert ch3.dose(speed=0.5, timeout=0.1, blocking=False) is True
+    assert global_lock.locked() is True
+    time.sleep(0.3)