diff --git a/spec-dtslint/operators/switchScan-spec.ts b/spec-dtslint/operators/switchScan-spec.ts
index 86196c1bfb2..41e03a703e3 100644
--- a/spec-dtslint/operators/switchScan-spec.ts
+++ b/spec-dtslint/operators/switchScan-spec.ts
@@ -9,16 +9,20 @@ it('should infer correctly when using a single type', () => {
   const o = of(1, 2, 3).pipe(switchScan((acc, v) => of(acc + v), 0)); // $ExpectType Observable<number>
 });
 
-it('should infer correctly when using a seed', () => {
-  const o = of(1, 2, 3).pipe(switchScan((acc, v) => of(acc + v), 0)); // $ExpectType Observable<number>
-});
-
 it('should infer correctly when using seed of a different type', () => {
-  const o = of(1, 2, 3).pipe(switchScan((acc: string, v: number) => of(acc + v), '0')); // $ExpectType Observable<string>
+  const o = of(1, 2, 3).pipe(switchScan((acc, v) => of(acc + v), '0')); // $ExpectType Observable<string>
 });
 
 it('should support a projector that takes an index', () => {
-  const o = of(1, 2, 3).pipe(switchScan((acc: boolean, v: number, index: number) => of(Boolean(v)), false)); // $ExpectType Observable<boolean>
+  const o = of(1, 2, 3).pipe(switchScan((acc, v, index) => of(Boolean(v)), false)); // $ExpectType Observable<boolean>
+});
+
+it('should support projecting to union types', () => {
+  const o = of(Math.random()).pipe(switchScan(n => n > 0.5 ? of(123) : of('test'), 0)); // $ExpectType Observable<string | number>
+});
+
+it('should use the inferred accumulator return type over the seed type', () => {
+  const o = of(1, 2, 3).pipe(switchScan(p => of(1), [])); // $ExpectType Observable<number>
 });
 
 it('should enforce types', () => {
@@ -29,10 +33,6 @@ it('should enforce the return type to be Observable', () => {
   const o = of(1, 2, 3).pipe(switchScan(p => p)); // $ExpectError
 });
 
-it('should enforce seed and the return type from accumulator', () => {
-  const o = of(1, 2, 3).pipe(switchScan(p => of(1), [])); // $ExpectError
-});
-
 it('should enforce seed and accumulator to have the same type', () => {
   const o = of(1, 2, 3).pipe(switchScan((acc, p) => of([...acc, p]))); // $ExpectError
 });
diff --git a/spec/operators/switchScan-spec.ts b/spec/operators/switchScan-spec.ts
index 83422906011..ee3ddaa29e3 100644
--- a/spec/operators/switchScan-spec.ts
+++ b/spec/operators/switchScan-spec.ts
@@ -8,14 +8,14 @@ declare function asDiagram(arg: string): Function;
 /** @test {switchScan} */
 describe('switchScan', () => {
   asDiagram('switchScan(i => 10*i\u2014\u201410*i\u2014\u201410*i\u2014|, 0)')
-  ('should map-and-flatten each item to an Observable while passing the seed', () => {
+  ('should map-and-flatten each item to an Observable while passing the accumulated value', () => {
     const e1 =    hot('--1-----3--5-------|');
     const e1subs =    '^                  !';
     const e2 =   cold('x-x-x|              ', {x: 10});
     const expected =  '--x-x-x-y-yz-z-z---|';
     const values = {x: 10, y: 40, z: 90};
 
-    const result = e1.pipe(switchScan((acc: number, x: string) => e2.pipe(map(i => i * Number(x) + acc)), 0));
+    const result = e1.pipe(switchScan((acc, x) => e2.pipe(map(i => i * Number(x) + acc)), 0));
 
     expectObservable(result).toBe(expected, values);
     expectSubscriptions(e1.subscriptions).toBe(e1subs);
@@ -61,7 +61,7 @@ describe('switchScan', () => {
     const e1subs =   '^                            !        ';
     const expected = '-----------a--b--c----f---g---h---i--|';
 
-    const observableLookup = { x: x, y: y };
+    const observableLookup = { x, y };
 
     const result = e1.pipe(switchScan((acc, value) => observableLookup[value], null));
 
diff --git a/src/internal/operators/switchScan.ts b/src/internal/operators/switchScan.ts
index 9d4215cecf1..0683d46ecfb 100644
--- a/src/internal/operators/switchScan.ts
+++ b/src/internal/operators/switchScan.ts
@@ -1,5 +1,5 @@
 import { Observable } from '../Observable';
-import { ObservableInput, OperatorFunction } from '../types';
+import { ObservableInput, ObservedValueOf, OperatorFunction } from '../types';
 import { Operator } from '../Operator';
 import { Subscriber } from '../Subscriber';
 import { switchMap } from './switchMap';
@@ -25,21 +25,21 @@ import { tap } from './tap';
  * @method switchScan
  * @owner Observable
  */
-export function switchScan<T, R>(
-  accumulator: (acc: R, value: T, index: number) => ObservableInput<R>,
+export function switchScan<T, R, O extends ObservableInput<any>>(
+  accumulator: (acc: R, value: T, index: number) => O,
   seed: R
-): OperatorFunction<T, R> {
+): OperatorFunction<T, ObservedValueOf<O>> {
   return (source: Observable<T>) => source.lift(new SwitchScanOperator(accumulator, seed));
 }
 
-class SwitchScanOperator<T, R> implements Operator<T, R> {
-  constructor(private accumulator: (acc: R, value: T, index: number) => ObservableInput<R>, private seed: R) { }
+class SwitchScanOperator<T, R, O extends ObservableInput<any>> implements Operator<T, O> {
+  constructor(private accumulator: (acc: R, value: T, index: number) => ObservableInput<O>, private seed: R) { }
 
-  call(subscriber: Subscriber<R>, source: any): any {
+  call(subscriber: Subscriber<O>, source: any): any {
     let seed: R = this.seed;
 
     return source.pipe(
-      switchMap((value: T, index: number): ObservableInput<R> => this.accumulator(seed, value, index)),
+      switchMap((value: T, index: number) => this.accumulator(seed, value, index)),
       tap((value: R) => seed = value),
     ).subscribe(subscriber);
   }