diff --git a/src/renderers/dom/client/ReactMount.js b/src/renderers/dom/client/ReactMount.js
index d7f24aaf386d0..c0e3c9ec00e2e 100644
--- a/src/renderers/dom/client/ReactMount.js
+++ b/src/renderers/dom/client/ReactMount.js
@@ -729,11 +729,11 @@ var ReactMount = {
     if (__DEV__) {
       var hostNode = ReactDOMComponentTree.getInstanceFromNode(container.firstChild);
       if (hostNode._debugID !== 0) {
-        ReactInstrumentation.debugTool.onHostOperation(
-          hostNode._debugID,
-          'mount',
-          markup.toString()
-        );
+        ReactInstrumentation.debugTool.onHostOperation({
+          instanceID: hostNode._debugID,
+          type: 'mount',
+          payload: markup.toString(),
+        });
       }
     }
   },
diff --git a/src/renderers/dom/client/utils/DOMChildrenOperations.js b/src/renderers/dom/client/utils/DOMChildrenOperations.js
index f13b16b3f6bc0..2200f2529ad6b 100644
--- a/src/renderers/dom/client/utils/DOMChildrenOperations.js
+++ b/src/renderers/dom/client/utils/DOMChildrenOperations.js
@@ -122,11 +122,11 @@ function replaceDelimitedText(openingComment, closingComment, stringText) {
   }
 
   if (__DEV__) {
-    ReactInstrumentation.debugTool.onHostOperation(
-      ReactDOMComponentTree.getInstanceFromNode(openingComment)._debugID,
-      'replace text',
-      stringText
-    );
+    ReactInstrumentation.debugTool.onHostOperation({
+      instanceID: ReactDOMComponentTree.getInstanceFromNode(openingComment)._debugID,
+      type: 'replace text',
+      payload: stringText,
+    });
   }
 }
 
@@ -135,19 +135,19 @@ if (__DEV__) {
   dangerouslyReplaceNodeWithMarkup = function(oldChild, markup, prevInstance) {
     Danger.dangerouslyReplaceNodeWithMarkup(oldChild, markup);
     if (prevInstance._debugID !== 0) {
-      ReactInstrumentation.debugTool.onHostOperation(
-        prevInstance._debugID,
-        'replace with',
-        markup.toString()
-      );
+      ReactInstrumentation.debugTool.onHostOperation({
+        instanceID: prevInstance._debugID,
+        type: 'replace with',
+        payload: markup.toString(),
+      });
     } else {
       var nextInstance = ReactDOMComponentTree.getInstanceFromNode(markup.node);
       if (nextInstance._debugID !== 0) {
-        ReactInstrumentation.debugTool.onHostOperation(
-          nextInstance._debugID,
-          'mount',
-          markup.toString()
-        );
+        ReactInstrumentation.debugTool.onHostOperation({
+          instanceID: nextInstance._debugID,
+          type: 'mount',
+          payload: markup.toString(),
+        });
       }
     }
   };
@@ -185,11 +185,11 @@ var DOMChildrenOperations = {
             getNodeAfter(parentNode, update.afterNode)
           );
           if (__DEV__) {
-            ReactInstrumentation.debugTool.onHostOperation(
-              parentNodeDebugID,
-              'insert child',
-              {toIndex: update.toIndex, content: update.content.toString()}
-            );
+            ReactInstrumentation.debugTool.onHostOperation({
+              instanceID: parentNodeDebugID,
+              type: 'insert child',
+              payload: {toIndex: update.toIndex, content: update.content.toString()},
+            });
           }
           break;
         case 'MOVE_EXISTING':
@@ -199,11 +199,11 @@ var DOMChildrenOperations = {
             getNodeAfter(parentNode, update.afterNode)
           );
           if (__DEV__) {
-            ReactInstrumentation.debugTool.onHostOperation(
-              parentNodeDebugID,
-              'move child',
-              {fromIndex: update.fromIndex, toIndex: update.toIndex}
-            );
+            ReactInstrumentation.debugTool.onHostOperation({
+              instanceID: parentNodeDebugID,
+              type: 'move child',
+              payload: {fromIndex: update.fromIndex, toIndex: update.toIndex},
+            });
           }
           break;
         case 'SET_MARKUP':
@@ -212,11 +212,11 @@ var DOMChildrenOperations = {
             update.content
           );
           if (__DEV__) {
-            ReactInstrumentation.debugTool.onHostOperation(
-              parentNodeDebugID,
-              'replace children',
-              update.content.toString()
-            );
+            ReactInstrumentation.debugTool.onHostOperation({
+              instanceID: parentNodeDebugID,
+              type: 'replace children',
+              payload: update.content.toString(),
+            });
           }
           break;
         case 'TEXT_CONTENT':
@@ -225,21 +225,21 @@ var DOMChildrenOperations = {
             update.content
           );
           if (__DEV__) {
-            ReactInstrumentation.debugTool.onHostOperation(
-              parentNodeDebugID,
-              'replace text',
-              update.content.toString()
-            );
+            ReactInstrumentation.debugTool.onHostOperation({
+              instanceID: parentNodeDebugID,
+              type: 'replace text',
+              payload: update.content.toString(),
+            });
           }
           break;
         case 'REMOVE_NODE':
           removeChild(parentNode, update.fromNode);
           if (__DEV__) {
-            ReactInstrumentation.debugTool.onHostOperation(
-              parentNodeDebugID,
-              'remove child',
-              {fromIndex: update.fromIndex}
-            );
+            ReactInstrumentation.debugTool.onHostOperation({
+              instanceID: parentNodeDebugID,
+              type: 'remove child',
+              payload: {fromIndex: update.fromIndex},
+            });
           }
           break;
       }
diff --git a/src/renderers/dom/shared/CSSPropertyOperations.js b/src/renderers/dom/shared/CSSPropertyOperations.js
index c3222c202f147..81c5319959687 100644
--- a/src/renderers/dom/shared/CSSPropertyOperations.js
+++ b/src/renderers/dom/shared/CSSPropertyOperations.js
@@ -193,11 +193,11 @@ var CSSPropertyOperations = {
    */
   setValueForStyles: function(node, styles, component) {
     if (__DEV__) {
-      ReactInstrumentation.debugTool.onHostOperation(
-        component._debugID,
-        'update styles',
-        styles
-      );
+      ReactInstrumentation.debugTool.onHostOperation({
+        instanceID: component._debugID,
+        type: 'update styles',
+        payload: styles,
+      });
     }
 
     var style = node.style;
diff --git a/src/renderers/dom/shared/DOMPropertyOperations.js b/src/renderers/dom/shared/DOMPropertyOperations.js
index 3d6cb7f59c751..895a7ec71ed2d 100644
--- a/src/renderers/dom/shared/DOMPropertyOperations.js
+++ b/src/renderers/dom/shared/DOMPropertyOperations.js
@@ -166,11 +166,11 @@ var DOMPropertyOperations = {
     if (__DEV__) {
       var payload = {};
       payload[name] = value;
-      ReactInstrumentation.debugTool.onHostOperation(
-        ReactDOMComponentTree.getInstanceFromNode(node)._debugID,
-        'update attribute',
-        payload
-      );
+      ReactInstrumentation.debugTool.onHostOperation({
+        instanceID: ReactDOMComponentTree.getInstanceFromNode(node)._debugID,
+        type: 'update attribute',
+        payload: payload,
+      });
     }
   },
 
@@ -187,11 +187,11 @@ var DOMPropertyOperations = {
     if (__DEV__) {
       var payload = {};
       payload[name] = value;
-      ReactInstrumentation.debugTool.onHostOperation(
-        ReactDOMComponentTree.getInstanceFromNode(node)._debugID,
-        'update attribute',
-        payload
-      );
+      ReactInstrumentation.debugTool.onHostOperation({
+        instanceID: ReactDOMComponentTree.getInstanceFromNode(node)._debugID,
+        type: 'update attribute',
+        payload: payload,
+      });
     }
   },
 
@@ -204,11 +204,11 @@ var DOMPropertyOperations = {
   deleteValueForAttribute: function(node, name) {
     node.removeAttribute(name);
     if (__DEV__) {
-      ReactInstrumentation.debugTool.onHostOperation(
-        ReactDOMComponentTree.getInstanceFromNode(node)._debugID,
-        'remove attribute',
-        name
-      );
+      ReactInstrumentation.debugTool.onHostOperation({
+        instanceID: ReactDOMComponentTree.getInstanceFromNode(node)._debugID,
+        type: 'remove attribute',
+        payload: name,
+      });
     }
   },
 
@@ -240,11 +240,11 @@ var DOMPropertyOperations = {
     }
 
     if (__DEV__) {
-      ReactInstrumentation.debugTool.onHostOperation(
-        ReactDOMComponentTree.getInstanceFromNode(node)._debugID,
-        'remove attribute',
-        name
-      );
+      ReactInstrumentation.debugTool.onHostOperation({
+        instanceID: ReactDOMComponentTree.getInstanceFromNode(node)._debugID,
+        type: 'remove attribute',
+        payload: name,
+      });
     }
   },
 
diff --git a/src/renderers/shared/ReactDebugTool.js b/src/renderers/shared/ReactDebugTool.js
index 31ef8c7073960..06581e5c74315 100644
--- a/src/renderers/shared/ReactDebugTool.js
+++ b/src/renderers/shared/ReactDebugTool.js
@@ -23,9 +23,9 @@ var warning = require('warning');
 
 import type { ReactElement } from 'ReactElementType';
 import type { DebugID } from 'ReactInstanceType';
+import type { Operation } from 'ReactHostOperationHistoryHook';
 
 type Hook = any;
-type Payload = mixed;
 
 type TimerType =
   'ctor' |
@@ -38,21 +38,6 @@ type TimerType =
   'componentDidUpdate' |
   'componentDidMount';
 
-type HostOperationType =
-  'mount' |
-  'insert child' |
-  'move child' |
-  'remove child' |
-  'replace children' |
-  'replace text' |
-  'replace with';
-
-type Operation = {
-  instanceID: DebugID,
-  type: string,
-  payload: Payload,
-};
-
 type Measurement = {
   timerType: TimerType,
   instanceID: DebugID,
@@ -378,9 +363,9 @@ var ReactDebugTool = {
   onEndProcessingChildContext(): void {
     emitEvent('onEndProcessingChildContext');
   },
-  onHostOperation(debugID: DebugID, type: HostOperationType, payload: Payload): void {
-    checkDebugID(debugID);
-    emitEvent('onHostOperation', debugID, type, payload);
+  onHostOperation(operation: Operation) {
+    checkDebugID(operation.instanceID);
+    emitEvent('onHostOperation', operation);
   },
   onSetState(): void {
     emitEvent('onSetState');
diff --git a/src/renderers/shared/hooks/ReactHostOperationHistoryHook.js b/src/renderers/shared/hooks/ReactHostOperationHistoryHook.js
index 48456caea93b9..675a00e732a18 100644
--- a/src/renderers/shared/hooks/ReactHostOperationHistoryHook.js
+++ b/src/renderers/shared/hooks/ReactHostOperationHistoryHook.js
@@ -7,22 +7,33 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule ReactHostOperationHistoryHook
+ * @flow
  */
 
 'use strict';
 
-var history = [];
+import type { DebugID } from 'ReactInstanceType';
+
+export type Operation = {instanceID: DebugID} & (
+  {type: 'mount', payload: string} |
+  {type: 'insert child', payload: {toIndex: number, content: string}} |
+  {type: 'move child', payload: {fromIndex: number, toIndex: number}} |
+  {type: 'replace children', payload: string} |
+  {type: 'replace text', payload: string} |
+  {type: 'replace with', payload: string} |
+  {type: 'update styles', payload: mixed /* Style Object */} |
+  {type: 'update attribute', payload: {[name: string]: string}} |
+  {type: 'remove attribute', payload: string}
+);
+
+var history: Array<Operation> = [];
 
 var ReactHostOperationHistoryHook = {
-  onHostOperation(debugID, type, payload) {
-    history.push({
-      instanceID: debugID,
-      type,
-      payload,
-    });
+  onHostOperation(operation: Operation) {
+    history.push(operation);
   },
 
-  clearHistory() {
+  clearHistory(): void {
     if (ReactHostOperationHistoryHook._preventClearing) {
       // Should only be used for tests.
       return;
@@ -31,7 +42,7 @@ var ReactHostOperationHistoryHook = {
     history = [];
   },
 
-  getHistory() {
+  getHistory(): Array<Operation> {
     return history;
   },
 };