diff --git a/src/lib/es5.d.ts b/src/lib/es5.d.ts
index 1f352cd6f3913..b777e534f2658 100644
--- a/src/lib/es5.d.ts
+++ b/src/lib/es5.d.ts
@@ -1253,7 +1253,7 @@ interface ArrayConstructor {
     (arrayLength?: number): any[];
     <T>(arrayLength: number): T[];
     <T>(...items: T[]): T[];
-    isArray(arg: any): arg is Array<any>;
+    isArray(arg: any): arg is ReadonlyArray<any>;
     readonly prototype: Array<any>;
 }
 
diff --git a/tests/baselines/reference/fixSignatureCaching.types b/tests/baselines/reference/fixSignatureCaching.types
index c3f4dbc152248..5ff819439e2e5 100644
--- a/tests/baselines/reference/fixSignatureCaching.types
+++ b/tests/baselines/reference/fixSignatureCaching.types
@@ -1109,9 +1109,9 @@ define(function () {
 >Array : ArrayConstructor
 
         Array.isArray : function (value) { return Object.prototype.toString.call(value) === '[object Array]'; };
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : (arg: any) => arg is ReadonlyArray<any>
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : (arg: any) => arg is ReadonlyArray<any>
 >function (value) { return Object.prototype.toString.call(value) === '[object Array]'; } : (value: any) => boolean
 >value : any
 >Object.prototype.toString.call(value) === '[object Array]' : boolean
diff --git a/tests/baselines/reference/isArray.errors.txt b/tests/baselines/reference/isArray.errors.txt
new file mode 100644
index 0000000000000..4846c42f3484f
--- /dev/null
+++ b/tests/baselines/reference/isArray.errors.txt
@@ -0,0 +1,33 @@
+tests/cases/compiler/isArray.ts(7,11): error TS2322: Type 'number' is not assignable to type 'string'.
+tests/cases/compiler/isArray.ts(17,11): error TS2322: Type 'number' is not assignable to type 'string'.
+tests/cases/compiler/isArray.ts(18,24): error TS2339: Property 'push' does not exist on type 'ReadonlyArray<number>'.
+
+
+==== tests/cases/compiler/isArray.ts (3 errors) ====
+    var maybeArray: number | number[];
+    
+    
+    if (Array.isArray(maybeArray)) {
+        maybeArray.length; // OK
+        maybeArray.push(0); // OK
+        const str: string = maybeArray[0]; // Expect error
+              ~~~
+!!! error TS2322: Type 'number' is not assignable to type 'string'.
+    }
+    else {
+        maybeArray.toFixed(); // OK
+    }
+    
+    var maybeReadonlyArray: number | ReadonlyArray<number>;
+    if (Array.isArray(maybeReadonlyArray)) {
+        maybeReadonlyArray.length; // OK
+        const num = maybeReadonlyArray[0]; // OK, expect typeof num = number
+        const str: string = maybeReadonlyArray[0]; // Expect error
+              ~~~
+!!! error TS2322: Type 'number' is not assignable to type 'string'.
+        maybeReadonlyArray.push(0); // Expect error
+                           ~~~~
+!!! error TS2339: Property 'push' does not exist on type 'ReadonlyArray<number>'.
+    } else {
+        maybeReadonlyArray.toFixed();
+    }
\ No newline at end of file
diff --git a/tests/baselines/reference/isArray.js b/tests/baselines/reference/isArray.js
index a90898fd65b26..d16a46f41a7a8 100644
--- a/tests/baselines/reference/isArray.js
+++ b/tests/baselines/reference/isArray.js
@@ -4,16 +4,40 @@ var maybeArray: number | number[];
 
 if (Array.isArray(maybeArray)) {
     maybeArray.length; // OK
+    maybeArray.push(0); // OK
+    const str: string = maybeArray[0]; // Expect error
 }
 else {
     maybeArray.toFixed(); // OK
+}
+
+var maybeReadonlyArray: number | ReadonlyArray<number>;
+if (Array.isArray(maybeReadonlyArray)) {
+    maybeReadonlyArray.length; // OK
+    const num = maybeReadonlyArray[0]; // OK, expect typeof num = number
+    const str: string = maybeReadonlyArray[0]; // Expect error
+    maybeReadonlyArray.push(0); // Expect error
+} else {
+    maybeReadonlyArray.toFixed();
 }
 
 //// [isArray.js]
 var maybeArray;
 if (Array.isArray(maybeArray)) {
     maybeArray.length; // OK
+    maybeArray.push(0); // OK
+    var str = maybeArray[0]; // Expect error
 }
 else {
     maybeArray.toFixed(); // OK
 }
+var maybeReadonlyArray;
+if (Array.isArray(maybeReadonlyArray)) {
+    maybeReadonlyArray.length; // OK
+    var num = maybeReadonlyArray[0]; // OK, expect typeof num = number
+    var str = maybeReadonlyArray[0]; // Expect error
+    maybeReadonlyArray.push(0); // Expect error
+}
+else {
+    maybeReadonlyArray.toFixed();
+}
diff --git a/tests/baselines/reference/isArray.symbols b/tests/baselines/reference/isArray.symbols
index b60123e6b2eae..37986dd7c2762 100644
--- a/tests/baselines/reference/isArray.symbols
+++ b/tests/baselines/reference/isArray.symbols
@@ -13,6 +13,15 @@ if (Array.isArray(maybeArray)) {
 >maybeArray.length : Symbol(Array.length, Decl(lib.d.ts, --, --))
 >maybeArray : Symbol(maybeArray, Decl(isArray.ts, 0, 3))
 >length : Symbol(Array.length, Decl(lib.d.ts, --, --))
+
+    maybeArray.push(0); // OK
+>maybeArray.push : Symbol(Array.push, Decl(lib.d.ts, --, --))
+>maybeArray : Symbol(maybeArray, Decl(isArray.ts, 0, 3))
+>push : Symbol(Array.push, Decl(lib.d.ts, --, --))
+
+    const str: string = maybeArray[0]; // Expect error
+>str : Symbol(str, Decl(isArray.ts, 6, 9))
+>maybeArray : Symbol(maybeArray, Decl(isArray.ts, 0, 3))
 }
 else {
     maybeArray.toFixed(); // OK
@@ -20,3 +29,36 @@ else {
 >maybeArray : Symbol(maybeArray, Decl(isArray.ts, 0, 3))
 >toFixed : Symbol(Number.toFixed, Decl(lib.d.ts, --, --))
 }
+
+var maybeReadonlyArray: number | ReadonlyArray<number>;
+>maybeReadonlyArray : Symbol(maybeReadonlyArray, Decl(isArray.ts, 12, 3))
+>ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.d.ts, --, --))
+
+if (Array.isArray(maybeReadonlyArray)) {
+>Array.isArray : Symbol(ArrayConstructor.isArray, Decl(lib.d.ts, --, --))
+>Array : Symbol(Array, Decl(lib.d.ts, --, --), Decl(lib.d.ts, --, --))
+>isArray : Symbol(ArrayConstructor.isArray, Decl(lib.d.ts, --, --))
+>maybeReadonlyArray : Symbol(maybeReadonlyArray, Decl(isArray.ts, 12, 3))
+
+    maybeReadonlyArray.length; // OK
+>maybeReadonlyArray.length : Symbol(ReadonlyArray.length, Decl(lib.d.ts, --, --))
+>maybeReadonlyArray : Symbol(maybeReadonlyArray, Decl(isArray.ts, 12, 3))
+>length : Symbol(ReadonlyArray.length, Decl(lib.d.ts, --, --))
+
+    const num = maybeReadonlyArray[0]; // OK, expect typeof num = number
+>num : Symbol(num, Decl(isArray.ts, 15, 9))
+>maybeReadonlyArray : Symbol(maybeReadonlyArray, Decl(isArray.ts, 12, 3))
+
+    const str: string = maybeReadonlyArray[0]; // Expect error
+>str : Symbol(str, Decl(isArray.ts, 16, 9))
+>maybeReadonlyArray : Symbol(maybeReadonlyArray, Decl(isArray.ts, 12, 3))
+
+    maybeReadonlyArray.push(0); // Expect error
+>maybeReadonlyArray : Symbol(maybeReadonlyArray, Decl(isArray.ts, 12, 3))
+
+} else {
+    maybeReadonlyArray.toFixed();
+>maybeReadonlyArray.toFixed : Symbol(Number.toFixed, Decl(lib.d.ts, --, --))
+>maybeReadonlyArray : Symbol(maybeReadonlyArray, Decl(isArray.ts, 12, 3))
+>toFixed : Symbol(Number.toFixed, Decl(lib.d.ts, --, --))
+}
diff --git a/tests/baselines/reference/isArray.types b/tests/baselines/reference/isArray.types
index bc452b12bef01..fe2b7a1ab2ee6 100644
--- a/tests/baselines/reference/isArray.types
+++ b/tests/baselines/reference/isArray.types
@@ -5,15 +5,28 @@ var maybeArray: number | number[];
 
 if (Array.isArray(maybeArray)) {
 >Array.isArray(maybeArray) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : (arg: any) => arg is ReadonlyArray<any>
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : (arg: any) => arg is ReadonlyArray<any>
 >maybeArray : number | number[]
 
     maybeArray.length; // OK
 >maybeArray.length : number
 >maybeArray : number[]
 >length : number
+
+    maybeArray.push(0); // OK
+>maybeArray.push(0) : number
+>maybeArray.push : (...items: number[]) => number
+>maybeArray : number[]
+>push : (...items: number[]) => number
+>0 : 0
+
+    const str: string = maybeArray[0]; // Expect error
+>str : string
+>maybeArray[0] : number
+>maybeArray : number[]
+>0 : 0
 }
 else {
     maybeArray.toFixed(); // OK
@@ -22,3 +35,46 @@ else {
 >maybeArray : number
 >toFixed : (fractionDigits?: number) => string
 }
+
+var maybeReadonlyArray: number | ReadonlyArray<number>;
+>maybeReadonlyArray : number | ReadonlyArray<number>
+>ReadonlyArray : ReadonlyArray<T>
+
+if (Array.isArray(maybeReadonlyArray)) {
+>Array.isArray(maybeReadonlyArray) : boolean
+>Array.isArray : (arg: any) => arg is ReadonlyArray<any>
+>Array : ArrayConstructor
+>isArray : (arg: any) => arg is ReadonlyArray<any>
+>maybeReadonlyArray : number | ReadonlyArray<number>
+
+    maybeReadonlyArray.length; // OK
+>maybeReadonlyArray.length : number
+>maybeReadonlyArray : ReadonlyArray<number>
+>length : number
+
+    const num = maybeReadonlyArray[0]; // OK, expect typeof num = number
+>num : number
+>maybeReadonlyArray[0] : number
+>maybeReadonlyArray : ReadonlyArray<number>
+>0 : 0
+
+    const str: string = maybeReadonlyArray[0]; // Expect error
+>str : string
+>maybeReadonlyArray[0] : number
+>maybeReadonlyArray : ReadonlyArray<number>
+>0 : 0
+
+    maybeReadonlyArray.push(0); // Expect error
+>maybeReadonlyArray.push(0) : any
+>maybeReadonlyArray.push : any
+>maybeReadonlyArray : ReadonlyArray<number>
+>push : any
+>0 : 0
+
+} else {
+    maybeReadonlyArray.toFixed();
+>maybeReadonlyArray.toFixed() : string
+>maybeReadonlyArray.toFixed : (fractionDigits?: number) => string
+>maybeReadonlyArray : number
+>toFixed : (fractionDigits?: number) => string
+}
diff --git a/tests/baselines/reference/malformedTags.types b/tests/baselines/reference/malformedTags.types
index c3af959586790..1ec38de5b9c28 100644
--- a/tests/baselines/reference/malformedTags.types
+++ b/tests/baselines/reference/malformedTags.types
@@ -6,7 +6,7 @@
  */
 var isArray = Array.isArray;
 >isArray : Function
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : (arg: any) => arg is ReadonlyArray<any>
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : (arg: any) => arg is ReadonlyArray<any>
 
diff --git a/tests/baselines/reference/parserharness.types b/tests/baselines/reference/parserharness.types
index 505cfe67c8203..cb0a049f746c9 100644
--- a/tests/baselines/reference/parserharness.types
+++ b/tests/baselines/reference/parserharness.types
@@ -3131,27 +3131,27 @@ module Harness {
 >identifier : any
 
             public normalizeToArray(arg: any) {
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => ReadonlyArray<any>
 >arg : any
 
                 if ((Array.isArray && Array.isArray(arg)) || arg instanceof Array)
 >(Array.isArray && Array.isArray(arg)) || arg instanceof Array : boolean
 >(Array.isArray && Array.isArray(arg)) : boolean
 >Array.isArray && Array.isArray(arg) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : (arg: any) => arg is ReadonlyArray<any>
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : (arg: any) => arg is ReadonlyArray<any>
 >Array.isArray(arg) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : (arg: any) => arg is ReadonlyArray<any>
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : (arg: any) => arg is ReadonlyArray<any>
 >arg : any
 >arg instanceof Array : boolean
 >arg : any
 >Array : ArrayConstructor
 
                     return arg;
->arg : any[]
+>arg : ReadonlyArray<any>
 
                 return [arg];
 >[arg] : any[]
@@ -3311,12 +3311,12 @@ module Harness {
 >others : any
 
                 others = this.normalizeToArray(others);
->others = this.normalizeToArray(others) : any[]
+>others = this.normalizeToArray(others) : ReadonlyArray<any>
 >others : any
->this.normalizeToArray(others) : any[]
->this.normalizeToArray : (arg: any) => any[]
+>this.normalizeToArray(others) : ReadonlyArray<any>
+>this.normalizeToArray : (arg: any) => ReadonlyArray<any>
 >this : this
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => ReadonlyArray<any>
 >others : any
 
                 for (var i = 0; i < others.length; i++) {
@@ -3365,12 +3365,12 @@ module Harness {
 >others : any
 
                 others = this.normalizeToArray(others);
->others = this.normalizeToArray(others) : any[]
+>others = this.normalizeToArray(others) : ReadonlyArray<any>
 >others : any
->this.normalizeToArray(others) : any[]
->this.normalizeToArray : (arg: any) => any[]
+>this.normalizeToArray(others) : ReadonlyArray<any>
+>this.normalizeToArray : (arg: any) => ReadonlyArray<any>
 >this : this
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => ReadonlyArray<any>
 >others : any
 
                 for (var i = 0; i < others.length; i++) {
@@ -3521,12 +3521,12 @@ module Harness {
 >others : any
 
                 others = this.normalizeToArray(others);
->others = this.normalizeToArray(others) : any[]
+>others = this.normalizeToArray(others) : ReadonlyArray<any>
 >others : any
->this.normalizeToArray(others) : any[]
->this.normalizeToArray : (arg: any) => any[]
+>this.normalizeToArray(others) : ReadonlyArray<any>
+>this.normalizeToArray : (arg: any) => ReadonlyArray<any>
 >this : this
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => ReadonlyArray<any>
 >others : any
 
                 for (var i = 0; i < others.length; i++) {
@@ -3577,12 +3577,12 @@ module Harness {
 >others : any
 
                 others = this.normalizeToArray(others);
->others = this.normalizeToArray(others) : any[]
+>others = this.normalizeToArray(others) : ReadonlyArray<any>
 >others : any
->this.normalizeToArray(others) : any[]
->this.normalizeToArray : (arg: any) => any[]
+>this.normalizeToArray(others) : ReadonlyArray<any>
+>this.normalizeToArray : (arg: any) => ReadonlyArray<any>
 >this : this
->normalizeToArray : (arg: any) => any[]
+>normalizeToArray : (arg: any) => ReadonlyArray<any>
 >others : any
 
                 for (var i = 0; i < others.length; i++) {
diff --git a/tests/baselines/reference/partiallyDiscriminantedUnions.types b/tests/baselines/reference/partiallyDiscriminantedUnions.types
index 911c949373ba6..455b57b2186b9 100644
--- a/tests/baselines/reference/partiallyDiscriminantedUnions.types
+++ b/tests/baselines/reference/partiallyDiscriminantedUnions.types
@@ -96,9 +96,9 @@ function isShape(s : Shapes): s is Shape {
     return !Array.isArray(s);
 >!Array.isArray(s) : boolean
 >Array.isArray(s) : boolean
->Array.isArray : (arg: any) => arg is any[]
+>Array.isArray : (arg: any) => arg is ReadonlyArray<any>
 >Array : ArrayConstructor
->isArray : (arg: any) => arg is any[]
+>isArray : (arg: any) => arg is ReadonlyArray<any>
 >s : Shapes
 }
 
diff --git a/tests/cases/compiler/isArray.ts b/tests/cases/compiler/isArray.ts
index dbdd875e21ecc..e08fa99663dd4 100644
--- a/tests/cases/compiler/isArray.ts
+++ b/tests/cases/compiler/isArray.ts
@@ -3,7 +3,19 @@ var maybeArray: number | number[];
 
 if (Array.isArray(maybeArray)) {
     maybeArray.length; // OK
+    maybeArray.push(0); // OK
+    const str: string = maybeArray[0]; // Expect error
 }
 else {
     maybeArray.toFixed(); // OK
+}
+
+var maybeReadonlyArray: number | ReadonlyArray<number>;
+if (Array.isArray(maybeReadonlyArray)) {
+    maybeReadonlyArray.length; // OK
+    const num = maybeReadonlyArray[0]; // OK, expect typeof num = number
+    const str: string = maybeReadonlyArray[0]; // Expect error
+    maybeReadonlyArray.push(0); // Expect error
+} else {
+    maybeReadonlyArray.toFixed();
 }
\ No newline at end of file