diff --git a/src/core/ReactCompositeComponent.js b/src/core/ReactCompositeComponent.js
index 914d3846e1..b66cf4900e 100644
--- a/src/core/ReactCompositeComponent.js
+++ b/src/core/ReactCompositeComponent.js
@@ -679,7 +679,7 @@ var ReactCompositeComponentMixin = {
       this._compositeLifeCycleState = CompositeLifeCycle.MOUNTING;
 
       this._defaultProps = this.getDefaultProps ? this.getDefaultProps() : null;
-      this._processProps(this.props);
+      this.props = this._processProps(this.props);
 
       if (this.__reactAutoBindMap) {
         this._bindAutoBindMethods();
@@ -853,12 +853,15 @@ var ReactCompositeComponentMixin = {
 
   /**
    * Processes props by setting default values for unspecified props and
-   * asserting that the props are valid.
+   * asserting that the props are valid. Does not mutate its argument; returns
+   * a new props object with defaults merged in.
    *
-   * @param {object} props
+   * @param {object} newProps
+   * @return {object}
    * @private
    */
-  _processProps: function(props) {
+  _processProps: function(newProps) {
+    var props = merge(newProps);
     var defaultProps = this._defaultProps;
     for (var propName in defaultProps) {
       if (typeof props[propName] === 'undefined') {
@@ -869,6 +872,7 @@ var ReactCompositeComponentMixin = {
     if (propTypes) {
       this._checkPropTypes(propTypes, props, ReactPropTypeLocations.prop);
     }
+    return props;
   },
 
   /**
@@ -920,8 +924,7 @@ var ReactCompositeComponentMixin = {
 
     var nextProps = this.props;
     if (this._pendingProps != null) {
-      nextProps = this._pendingProps;
-      this._processProps(nextProps);
+      nextProps = this._processProps(this._pendingProps);
       this._pendingProps = null;
 
       this._compositeLifeCycleState = CompositeLifeCycle.RECEIVING_PROPS;
diff --git a/src/core/__tests__/ReactCompositeComponent-test.js b/src/core/__tests__/ReactCompositeComponent-test.js
index 6866bcd2b2..1f2f8dd52b 100644
--- a/src/core/__tests__/ReactCompositeComponent-test.js
+++ b/src/core/__tests__/ReactCompositeComponent-test.js
@@ -213,6 +213,47 @@ describe('ReactCompositeComponent', function() {
     reactComponentExpect(instance3).scalarPropsEqual({key: null});
   });
 
+  it('should not mutate passed-in props object', function() {
+    var Component = React.createClass({
+      getDefaultProps: function() {
+        return {key: 'testKey'};
+      },
+      render: function() {
+        return <span />;
+      }
+    });
+
+    var inputProps = {};
+    var instance1 = Component(inputProps);
+    ReactTestUtils.renderIntoDocument(instance1);
+    expect(instance1.props.key).toBe('testKey');
+
+    // We don't mutate the input, just in case the caller wants to do something
+    // with it after using it to instantiate a component
+    expect(inputProps.key).not.toBeDefined();
+  });
+
+  it('should use default prop value when removing a key', function() {
+    var Component = React.createClass({
+      getDefaultProps: function() {
+        return {fruit: 'persimmon'};
+      },
+      render: function() {
+        return <span />;
+      }
+    });
+
+    var container = document.createElement('div');
+    var instance = React.renderComponent(
+      <Component fruit="mango" />,
+      container
+    );
+    expect(instance.props.fruit).toBe('mango');
+
+    React.renderComponent(<Component />, container);
+    expect(instance.props.fruit).toBe('persimmon');
+  });
+
   it('should normalize props with default values', function() {
     var Component = React.createClass({
       propTypes: {key: ReactPropTypes.string.isRequired},