diff --git a/src/core/ReactCompositeComponent.js b/src/core/ReactCompositeComponent.js
index e97b5439fc..df9e5bbd78 100644
--- a/src/core/ReactCompositeComponent.js
+++ b/src/core/ReactCompositeComponent.js
@@ -326,8 +326,9 @@ function validateMethodOverride(proto, name) {
 function validateLifeCycleOnReplaceState(instance) {
   var compositeLifeCycleState = instance._compositeLifeCycleState;
   invariant(
-    instance.isMounted(),
-    'replaceState(...): Can only update a mounted component.'
+    instance.isMounted() ||
+      compositeLifeCycleState === CompositeLifeCycle.MOUNTING,
+    'replaceState(...): Can only update a mounted or mounting component.'
   );
   invariant(
     compositeLifeCycleState !== CompositeLifeCycle.RECEIVING_STATE &&
@@ -480,6 +481,17 @@ var ReactCompositeComponentMixin = {
       ReactCurrentOwner.getDepth() + 1 : 0;
   },
 
+  /**
+   * Checks whether or not this composite component is mounted.
+   * @return {boolean} True if mounted, false otherwise.
+   * @protected
+   * @final
+   */
+  isMounted: function() {
+    return ReactComponent.Mixin.isMounted.call(this) &&
+      this._compositeLifeCycleState !== CompositeLifeCycle.MOUNTING;
+  },
+
   /**
    * Initializes the component, renders markup, and registers event listeners.
    *
@@ -756,8 +768,10 @@ var ReactCompositeComponentMixin = {
   forceUpdate: function() {
     var compositeLifeCycleState = this._compositeLifeCycleState;
     invariant(
-      this.isMounted(),
-      'forceUpdate(...): Can only force an update on mounted components.'
+      this.isMounted() ||
+        compositeLifeCycleState === CompositeLifeCycle.MOUNTING,
+      'forceUpdate(...): Can only force an update on mounted or mounting ' +
+        'components.'
     );
     invariant(
       compositeLifeCycleState !== CompositeLifeCycle.RECEIVING_STATE &&
diff --git a/src/core/__tests__/ReactComponent-test.js b/src/core/__tests__/ReactComponent-test.js
index a753ce0c89..5b1d96a9e2 100644
--- a/src/core/__tests__/ReactComponent-test.js
+++ b/src/core/__tests__/ReactComponent-test.js
@@ -95,4 +95,23 @@ describe('ReactComponent', function() {
     ReactTestUtils.renderIntoDocument(instance);
   });
 
+  it('should correctly determine if a component is mounted', function() {
+    var Component = React.createClass({
+      componentWillMount: function() {
+        expect(this.isMounted()).toBeFalsy();
+      },
+      componentDidMount: function() {
+        expect(this.isMounted()).toBeTruthy();
+      },
+      render: function() {
+        return <div/>;
+      }
+    });
+
+    var instance = <Component />;
+
+    expect(instance.isMounted()).toBeFalsy();
+    ReactTestUtils.renderIntoDocument(instance);
+    expect(instance.isMounted()).toBeTruthy();
+  });
 });
diff --git a/src/core/__tests__/ReactCompositeComponent-test.js b/src/core/__tests__/ReactCompositeComponent-test.js
index 6bfd0b03d5..c9aad6498a 100644
--- a/src/core/__tests__/ReactCompositeComponent-test.js
+++ b/src/core/__tests__/ReactCompositeComponent-test.js
@@ -308,7 +308,7 @@ describe('ReactCompositeComponent', function() {
       instance.forceUpdate();
     }).toThrow(
       'Invariant Violation: forceUpdate(...): Can only force an update on ' +
-      'mounted components.'
+      'mounted or mounting components.'
     );
 
     React.renderComponent(instance, container);
@@ -321,7 +321,7 @@ describe('ReactCompositeComponent', function() {
       instance.forceUpdate();
     }).toThrow(
       'Invariant Violation: forceUpdate(...): Can only force an update on ' +
-      'mounted components.'
+      'mounted or mounting components.'
     );
   });