diff --git a/src/renderers/dom/client/ReactReconcileTransaction.js b/src/renderers/dom/client/ReactReconcileTransaction.js
index 833e415f802..a14cb71cbfa 100644
--- a/src/renderers/dom/client/ReactReconcileTransaction.js
+++ b/src/renderers/dom/client/ReactReconcileTransaction.js
@@ -175,7 +175,7 @@ var Mixin = {
 };
 
 
-Object.assign(ReactReconcileTransaction.prototype, Transaction.Mixin, Mixin);
+Object.assign(ReactReconcileTransaction.prototype, Transaction, Mixin);
 
 PooledClass.addPoolingTo(ReactReconcileTransaction);
 
diff --git a/src/renderers/dom/server/ReactServerRenderingTransaction.js b/src/renderers/dom/server/ReactServerRenderingTransaction.js
index ddb93b6311d..0a8936bbb79 100644
--- a/src/renderers/dom/server/ReactServerRenderingTransaction.js
+++ b/src/renderers/dom/server/ReactServerRenderingTransaction.js
@@ -88,7 +88,7 @@ var Mixin = {
 
 Object.assign(
   ReactServerRenderingTransaction.prototype,
-  Transaction.Mixin,
+  Transaction,
   Mixin
 );
 
diff --git a/src/renderers/dom/server/ReactServerUpdateQueue.js b/src/renderers/dom/server/ReactServerUpdateQueue.js
index 4033339055b..b1e6a54c2ea 100644
--- a/src/renderers/dom/server/ReactServerUpdateQueue.js
+++ b/src/renderers/dom/server/ReactServerUpdateQueue.js
@@ -13,9 +13,11 @@
 'use strict';
 
 var ReactUpdateQueue = require('ReactUpdateQueue');
-var Transaction = require('Transaction');
+
 var warning = require('warning');
 
+import type { Transaction } from 'Transaction';
+
 function warnNoop(publicInstance: ReactComponent<any, any, any>, callerName: string) {
   if (__DEV__) {
     var constructor = publicInstance.constructor;
@@ -39,7 +41,7 @@ function warnNoop(publicInstance: ReactComponent<any, any, any>, callerName: str
  * @param {Transaction} transaction
  */
 class ReactServerUpdateQueue {
-  /* :: transaction: Transaction; */
+  transaction: Transaction;
 
   constructor(transaction: Transaction) {
     this.transaction = transaction;
diff --git a/src/renderers/native/ReactNativeReconcileTransaction.js b/src/renderers/native/ReactNativeReconcileTransaction.js
index ec9b969b902..ca2e5a03496 100644
--- a/src/renderers/native/ReactNativeReconcileTransaction.js
+++ b/src/renderers/native/ReactNativeReconcileTransaction.js
@@ -109,7 +109,7 @@ var Mixin = {
 
 Object.assign(
   ReactNativeReconcileTransaction.prototype,
-  Transaction.Mixin,
+  Transaction,
   ReactNativeReconcileTransaction,
   Mixin
 );
diff --git a/src/renderers/shared/stack/reconciler/ReactDefaultBatchingStrategy.js b/src/renderers/shared/stack/reconciler/ReactDefaultBatchingStrategy.js
index 4022775d0da..b5db558d400 100644
--- a/src/renderers/shared/stack/reconciler/ReactDefaultBatchingStrategy.js
+++ b/src/renderers/shared/stack/reconciler/ReactDefaultBatchingStrategy.js
@@ -36,7 +36,7 @@ function ReactDefaultBatchingStrategyTransaction() {
 
 Object.assign(
   ReactDefaultBatchingStrategyTransaction.prototype,
-  Transaction.Mixin,
+  Transaction,
   {
     getTransactionWrappers: function() {
       return TRANSACTION_WRAPPERS;
diff --git a/src/renderers/shared/stack/reconciler/ReactUpdates.js b/src/renderers/shared/stack/reconciler/ReactUpdates.js
index f1fdb05e1fc..d605c5881e2 100644
--- a/src/renderers/shared/stack/reconciler/ReactUpdates.js
+++ b/src/renderers/shared/stack/reconciler/ReactUpdates.js
@@ -75,7 +75,7 @@ function ReactUpdatesFlushTransaction() {
 
 Object.assign(
   ReactUpdatesFlushTransaction.prototype,
-  Transaction.Mixin,
+  Transaction,
   {
     getTransactionWrappers: function() {
       return TRANSACTION_WRAPPERS;
@@ -92,7 +92,7 @@ Object.assign(
     perform: function(method, scope, a) {
       // Essentially calls `this.reconcileTransaction.perform(method, scope, a)`
       // with this transaction's wrappers around it.
-      return Transaction.Mixin.perform.call(
+      return Transaction.perform.call(
         this,
         this.reconcileTransaction.perform,
         this.reconcileTransaction,
diff --git a/src/renderers/shared/utils/Transaction.js b/src/renderers/shared/utils/Transaction.js
index 84b55a25c8e..b8bee0b169c 100644
--- a/src/renderers/shared/utils/Transaction.js
+++ b/src/renderers/shared/utils/Transaction.js
@@ -7,12 +7,15 @@
  * of patent rights can be found in the PATENTS file in the same directory.
  *
  * @providesModule Transaction
+ * @flow
  */
 
 'use strict';
 
 var invariant = require('invariant');
 
+var OBSERVED_ERROR = {};
+
 /**
  * `Transaction` creates a black box that is able to wrap any method such that
  * certain invariants are maintained before and after the method is invoked
@@ -74,7 +77,7 @@ var invariant = require('invariant');
  *
  * @class Transaction
  */
-var Mixin = {
+var TransactionImpl = {
   /**
    * Sets up this instance so that it is prepared for collecting metrics. Does
    * so such that this setup method may be used on an instance that is already
@@ -82,7 +85,7 @@ var Mixin = {
    * That can be useful if you decide to make your subclass of this mixin a
    * "PooledClass".
    */
-  reinitializeTransaction: function() {
+  reinitializeTransaction: function(): void {
     this.transactionWrappers = this.getTransactionWrappers();
     if (this.wrapperInitData) {
       this.wrapperInitData.length = 0;
@@ -100,7 +103,7 @@ var Mixin = {
    */
   getTransactionWrappers: null,
 
-  isInTransaction: function() {
+  isInTransaction: function(): bool {
     return !!this._isInTransaction;
   },
 
@@ -121,7 +124,13 @@ var Mixin = {
    *
    * @return {*} Return value from `method`.
    */
-  perform: function(method, scope, a, b, c, d, e, f) {
+  perform: function<
+    A, B, C, D, E, F, G,
+    T: (a: A, b: B, c: C, d: D, e: E, f: F) => G // eslint-disable-line space-before-function-paren
+  >(
+    method: T, scope: any,
+    a: A, b: B, c: C, d: D, e: E, f: F,
+  ): G {
     invariant(
       !this.isInTransaction(),
       'Transaction.perform(...): Cannot initialize a transaction when there ' +
@@ -160,7 +169,7 @@ var Mixin = {
     return ret;
   },
 
-  initializeAll: function(startIndex) {
+  initializeAll: function(startIndex: number): void {
     var transactionWrappers = this.transactionWrappers;
     for (var i = startIndex; i < transactionWrappers.length; i++) {
       var wrapper = transactionWrappers[i];
@@ -169,12 +178,12 @@ var Mixin = {
         // OBSERVED_ERROR state before overwriting it with the real return value
         // of initialize -- if it's still set to OBSERVED_ERROR in the finally
         // block, it means wrapper.initialize threw.
-        this.wrapperInitData[i] = Transaction.OBSERVED_ERROR;
+        this.wrapperInitData[i] = OBSERVED_ERROR;
         this.wrapperInitData[i] = wrapper.initialize ?
           wrapper.initialize.call(this) :
           null;
       } finally {
-        if (this.wrapperInitData[i] === Transaction.OBSERVED_ERROR) {
+        if (this.wrapperInitData[i] === OBSERVED_ERROR) {
           // The initializer for wrapper i threw an error; initialize the
           // remaining wrappers but silence any exceptions from them to ensure
           // that the first error is the one to bubble up.
@@ -193,7 +202,7 @@ var Mixin = {
    * (`close`rs that correspond to initializers that failed will not be
    * invoked).
    */
-  closeAll: function(startIndex) {
+  closeAll: function(startIndex: number): void {
     invariant(
       this.isInTransaction(),
       'Transaction.closeAll(): Cannot close transaction when none are open.'
@@ -209,7 +218,7 @@ var Mixin = {
         // close -- if it's still set to true in the finally block, it means
         // wrapper.close threw.
         errorThrown = true;
-        if (initData !== Transaction.OBSERVED_ERROR && wrapper.close) {
+        if (initData !== OBSERVED_ERROR && wrapper.close) {
           wrapper.close.call(this, initData);
         }
         errorThrown = false;
@@ -229,15 +238,6 @@ var Mixin = {
   },
 };
 
-var Transaction = {
-
-  Mixin: Mixin,
-
-  /**
-   * Token to look for to determine if an error occurred.
-   */
-  OBSERVED_ERROR: {},
-
-};
+export type Transaction = typeof TransactionImpl;
 
-module.exports = Transaction;
+module.exports = TransactionImpl;
diff --git a/src/renderers/shared/utils/__tests__/Transaction-test.js b/src/renderers/shared/utils/__tests__/Transaction-test.js
index d3a222ed938..92c567b6f3a 100644
--- a/src/renderers/shared/utils/__tests__/Transaction-test.js
+++ b/src/renderers/shared/utils/__tests__/Transaction-test.js
@@ -45,7 +45,7 @@ describe('Transaction', function() {
       this.secondCloseParam = INIT_ERRORED;  // WILL be set to something else
       this.lastCloseParam = INIT_ERRORED;    // WON'T be set to something else
     };
-    Object.assign(TestTransaction.prototype, Transaction.Mixin);
+    Object.assign(TestTransaction.prototype, Transaction);
     TestTransaction.prototype.getTransactionWrappers = function() {
       return [
         {
@@ -96,7 +96,7 @@ describe('Transaction', function() {
       this.secondCloseParam = INIT_ERRORED;  // WILL be set to something else
       this.lastCloseParam = INIT_ERRORED;    // WILL be set to something else
     };
-    Object.assign(TestTransaction.prototype, Transaction.Mixin);
+    Object.assign(TestTransaction.prototype, Transaction);
     TestTransaction.prototype.getTransactionWrappers = function() {
       return [
         {
@@ -157,7 +157,7 @@ describe('Transaction', function() {
       this.secondCloseParam = INIT_ERRORED;  // WILL be set to something else
       this.lastCloseParam = INIT_ERRORED;    // WILL be set to something else
     };
-    Object.assign(TestTransaction.prototype, Transaction.Mixin);
+    Object.assign(TestTransaction.prototype, Transaction);
     // Now, none of the close/inits throw, but the operation we wrap will throw.
     TestTransaction.prototype.getTransactionWrappers = function() {
       return [
@@ -221,7 +221,7 @@ describe('Transaction', function() {
     var TestTransaction = function() {
       this.reinitializeTransaction();
     };
-    Object.assign(TestTransaction.prototype, Transaction.Mixin);
+    Object.assign(TestTransaction.prototype, Transaction);
     var exceptionMsg = 'This exception should throw.';
     TestTransaction.prototype.getTransactionWrappers = function() {
       return [
@@ -250,7 +250,7 @@ describe('Transaction', function() {
       this.reinitializeTransaction();
       this.firstCloseParam = INIT_ERRORED; // WILL be set to something else
     };
-    Object.assign(TestTransaction.prototype, Transaction.Mixin);
+    Object.assign(TestTransaction.prototype, Transaction);
     TestTransaction.prototype.getTransactionWrappers = function() {
       return [
         {
@@ -278,7 +278,7 @@ describe('Transaction', function() {
     var NestedTransaction = function() {
       this.reinitializeTransaction();
     };
-    Object.assign(NestedTransaction.prototype, Transaction.Mixin);
+    Object.assign(NestedTransaction.prototype, Transaction);
     NestedTransaction.prototype.getTransactionWrappers = function() {
       return [
         {
diff --git a/src/renderers/testing/ReactTestReconcileTransaction.js b/src/renderers/testing/ReactTestReconcileTransaction.js
index 0929103fa12..0fcd0daa50b 100644
--- a/src/renderers/testing/ReactTestReconcileTransaction.js
+++ b/src/renderers/testing/ReactTestReconcileTransaction.js
@@ -114,7 +114,7 @@ var Mixin = {
 
 Object.assign(
   ReactTestReconcileTransaction.prototype,
-  Transaction.Mixin,
+  Transaction,
   ReactTestReconcileTransaction,
   Mixin
 );