diff --git a/promises-a-plus/promises-a-plus-tests.ts b/promises-a-plus/promises-a-plus-tests.ts
new file mode 100644
index 000000000..5b10b1b6e
--- /dev/null
+++ b/promises-a-plus/promises-a-plus-tests.ts
@@ -0,0 +1,41 @@
+/// <reference path="promises-a-plus.d.ts"/>
+/// <reference path="../rx.js/rx.async.d.ts"/>
+
+var thenNum: PromisesAPlus.Thenable<number>; 
+var thenStr: PromisesAPlus.Thenable<string>; 
+var thenBool: PromisesAPlus.Thenable<boolean>; 
+
+var impl: PromisesAPlus.PromiseImpl;
+
+declare class PromiseImpl<R> {
+	constructor(resolver: (resolvePromise: (value: R) => void, rejectPromise: (reason: any) => void) => void);
+
+	then<U>(onFulfill: (value: R) => PromiseImpl<U>, onReject: (error: any) => PromiseImpl<U>): PromiseImpl<U>;
+	then<U>(onFulfill: (value: R) => PromiseImpl<U>, onReject?: (error: any) => U): PromiseImpl<U>;
+	then<U>(onFulfill: (value: R) => U, onReject: (error: any) => PromiseImpl<U>): PromiseImpl<U>;
+	then<U>(onFulfill?: (value: R) => U, onReject?: (error: any) => U): PromiseImpl<U>;
+}
+
+function testCompatibleWithPromiseImpl() {
+	var pNum: PromiseImpl<number> = thenNum;
+
+	thenNum = pNum;
+
+	impl = PromiseImpl;
+}
+
+function testCompatibleWithRxJS() {
+	// from spec to Rx
+	var rxThenNum: Rx.IPromise<number> = thenNum;
+	var rxThenStr: Rx.IPromise<string> = thenStr;
+
+	// from Rx to spec
+	thenNum = rxThenNum;
+	thenStr = rxThenStr;
+
+	var obsNum: Rx.Observable<number>;
+
+	thenNum = obsNum.toPromise<PromisesAPlus.Thenable<number>>(impl);
+	thenNum = obsNum.toPromise(impl);
+	obsNum.toPromise(PromiseImpl);
+}
diff --git a/promises-a-plus/promises-a-plus.d.ts b/promises-a-plus/promises-a-plus.d.ts
new file mode 100644
index 000000000..7a3bee776
--- /dev/null
+++ b/promises-a-plus/promises-a-plus.d.ts
@@ -0,0 +1,16 @@
+declare module PromisesAPlus {
+	interface PromiseCtor {
+		<T>(resolver: (resolvePromise: (value: T) => void, rejectPromise: (reason: any) => void) => void): Thenable<T>;
+	}
+
+	interface PromiseImpl {
+		new <T>(resolver: (resolvePromise: (value: T) => void, rejectPromise: (reason: any) => void) => void): Thenable<T>;
+	}
+
+	interface Thenable<R> {
+		then<U>(onFulfill: (value: R) => Thenable<U>, onReject: (error: any) => Thenable<U>): Thenable<U>;
+		then<U>(onFulfill: (value: R) => Thenable<U>, onReject?: (error: any) => U): Thenable<U>;
+		then<U>(onFulfill: (value: R) => U, onReject: (error: any) => Thenable<U>): Thenable<U>;
+		then<U>(onFulfill?: (value: R) => U, onReject?: (error: any) => U): Thenable<U>;
+	}
+}
diff --git a/rx.js/rx.async.d.ts b/rx.js/rx.async.d.ts
index cdd93b9a8..7238acafb 100644
--- a/rx.js/rx.async.d.ts
+++ b/rx.js/rx.async.d.ts
@@ -104,9 +104,9 @@ declare module Rx {
 	 * Promise A+
 	 */
 	export interface IPromise<T> {
-		then<R>(onFulfilled?: (value: T) => IPromise<R>, onRejected?: (reason: any) => IPromise<R>): IPromise<R>;
-		then<R>(onFulfilled?: (value: T) => IPromise<R>, onRejected?: (reason: any) => R): IPromise<R>;
-		then<R>(onFulfilled?: (value: T) => R, onRejected?: (reason: any) => IPromise<R>): IPromise<R>;
+		then<R>(onFulfilled: (value: T) => IPromise<R>, onRejected: (reason: any) => IPromise<R>): IPromise<R>;
+		then<R>(onFulfilled: (value: T) => IPromise<R>, onRejected?: (reason: any) => R): IPromise<R>;
+		then<R>(onFulfilled: (value: T) => R, onRejected: (reason: any) => IPromise<R>): IPromise<R>;
 		then<R>(onFulfilled?: (value: T) => R, onRejected?: (reason: any) => R): IPromise<R>;
 	}
 }