diff --git a/spec/operators/reduce-spec.ts b/spec/operators/reduce-spec.ts
index 1c228343d5..427342a77e 100644
--- a/spec/operators/reduce-spec.ts
+++ b/spec/operators/reduce-spec.ts
@@ -301,24 +301,85 @@ describe('Observable.prototype.reduce', () => {
   });
 
   it('should accept T typed reducers', () => {
+    type(() => {
+      let a: Rx.Observable<{ a: number; b: string }>;
+      const reduced = a.reduce((acc, value) => {
+        value.a = acc.a;
+        value.b = acc.b;
+        return acc;
+      });
+
+      reduced.subscribe(r => {
+        r.a.toExponential();
+        r.b.toLowerCase();
+      });
+    });
+  });
+
+  it('should accept R typed reducers when R is assignable to T', () => {
     type(() => {
       let a: Rx.Observable<{ a?: number; b?: string }>;
-      a.reduce((acc, value) => {
+      const reduced = a.reduce((acc, value) => {
         value.a = acc.a;
         value.b = acc.b;
         return acc;
       }, {});
+
+      reduced.subscribe(r => {
+        r.a.toExponential();
+        r.b.toLowerCase();
+      });
+    });
+  });
+
+  it('should accept R typed reducers when R is not assignable to T', () => {
+    type(() => {
+      let a: Rx.Observable<{ a: number; b: string }>;
+      const seed = {
+        as: [1],
+        bs: ['a']
+      };
+      const reduced = a.reduce((acc, value) => {
+        acc.as.push(value.a);
+        acc.bs.push(value.b);
+        return acc;
+      }, seed);
+
+      reduced.subscribe(r => {
+        r.as[0].toExponential();
+        r.bs[0].toLowerCase();
+      });
     });
   });
 
-  it('should accept R typed reducers', () => {
+  it('should accept R typed reducers and reduce to type R', () => {
     type(() => {
       let a: Rx.Observable<{ a: number; b: string }>;
-      a.reduce<{ a?: number; b?: string }>((acc, value) => {
+      const reduced = a.reduce<{ a?: number; b?: string }>((acc, value) => {
         value.a = acc.a;
         value.b = acc.b;
         return acc;
       }, {});
+
+      reduced.subscribe(r => {
+        r.a.toExponential();
+        r.b.toLowerCase();
+      });
+    });
+  });
+
+  it('should accept array of R typed reducers and reduce to array of R', () => {
+    type(() => {
+      let a: Rx.Observable<number>;
+      const reduced = a.reduce((acc, cur) => {
+        console.log(acc);
+        acc.push(cur.toString());
+        return acc;
+      }, [] as string[]);
+
+      reduced.subscribe(rs => {
+        rs[0].toLowerCase();
+      });
     });
   });
-});
\ No newline at end of file
+});
diff --git a/src/operator/reduce.ts b/src/operator/reduce.ts
index 10b98b62b2..aefb943955 100644
--- a/src/operator/reduce.ts
+++ b/src/operator/reduce.ts
@@ -3,9 +3,9 @@ import { Operator } from '../Operator';
 import { Subscriber } from '../Subscriber';
 
 /* tslint:disable:max-line-length */
+export function reduce<T>(this: Observable<T>, accumulator: (acc: T[], value: T, index: number) => T[], seed: T[]): Observable<T[]>;
 export function reduce<T>(this: Observable<T>, accumulator: (acc: T, value: T, index: number) => T, seed?: T): Observable<T>;
-export function reduce<T>(this: Observable<T>, accumulator: (acc: T[], value: T, index: number) => T[], seed?: T[]): Observable<T[]>;
-export function reduce<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed?: R): Observable<R>;
+export function reduce<T, R>(this: Observable<T>, accumulator: (acc: R, value: T, index: number) => R, seed: R): Observable<R>;
 /* tslint:enable:max-line-length */
 
 /**