diff --git a/src/compiler/scanner.ts b/src/compiler/scanner.ts
index d633e4926b2..7eaca34a212 100644
--- a/src/compiler/scanner.ts
+++ b/src/compiler/scanner.ts
@@ -832,12 +832,13 @@ namespace ts {
 
     /* @internal */
     export function isIdentifierText(name: string, languageVersion: ScriptTarget | undefined): boolean {
-        if (!isIdentifierStart(name.charCodeAt(0), languageVersion)) {
+        let ch = codePointAt(name, 0);
+        if (!isIdentifierStart(ch, languageVersion)) {
             return false;
         }
 
-        for (let i = 1; i < name.length; i++) {
-            if (!isIdentifierPart(name.charCodeAt(i), languageVersion)) {
+        for (let i = charSize(ch); i < name.length; i += charSize(ch)) {
+            if (!isIdentifierPart(ch = codePointAt(name, i), languageVersion)) {
                 return false;
             }
         }
@@ -1870,13 +1871,6 @@ namespace ts {
             }
         }
 
-        function charSize(ch: number) {
-            if (ch > 0x10000) {
-                return 2;
-            }
-            return 1;
-        }
-
         function reScanGreaterToken(): SyntaxKind {
             if (token === SyntaxKind.GreaterThanToken) {
                 if (text.charCodeAt(pos) === CharacterCodes.greaterThan) {
@@ -2238,4 +2232,12 @@ namespace ts {
         }
         return first;
     };
+
+    /* @internal */
+    function charSize(ch: number) {
+        if (ch > 0x10000) {
+            return 2;
+        }
+        return 1;
+    }
 }
diff --git a/tests/baselines/reference/extendedUnicodePlaneIdentifiers.js b/tests/baselines/reference/extendedUnicodePlaneIdentifiers.js
index 73271b8b671..7a5a5d9cc04 100644
--- a/tests/baselines/reference/extendedUnicodePlaneIdentifiers.js
+++ b/tests/baselines/reference/extendedUnicodePlaneIdentifiers.js
@@ -2,9 +2,64 @@
 const 𝑚 = 4;
 const 𝑀 = 5;
 console.log(𝑀 + 𝑚); // 9
+
+// lower 8 bits look like 'a'
+const ၡ = 6;
+console.log(ၡ ** ၡ);
+
+// lower 8 bits aren't a valid unicode character
+const ဒ = 7;
+console.log(ဒ ** ဒ);
+
+// a mix, for good measure
+const ဒၡ𝑀 = 7;
+console.log(ဒၡ𝑀 ** ဒၡ𝑀);
+
+const ၡ𝑀ဒ = 7;
+console.log(ၡ𝑀ဒ ** ၡ𝑀ဒ);
+
+const 𝑀ဒၡ = 7;
+console.log(𝑀ဒၡ ** 𝑀ဒၡ);
+
+const 𝓱𝓮𝓵𝓵𝓸 = "𝔀𝓸𝓻𝓵𝓭";
+
+const Ɐⱱ = "ok"; // BMP
+
+const 𓀸𓀹𓀺 = "ok"; // SMP
+
+const 𡚭𡚮𡚯 = "ok"; // SIP
+
+const 𡚭𓀺ⱱ𝓮 = "ok";
+
+const 𓀺ⱱ𝓮𡚭 = "ok";
+
+const ⱱ𝓮𡚭𓀺 = "ok";
+
+const 𝓮𡚭𓀺ⱱ = "ok";
 
 
 //// [extendedUnicodePlaneIdentifiers.js]
 const 𝑚 = 4;
 const 𝑀 = 5;
 console.log(𝑀 + 𝑚); // 9
+// lower 8 bits look like 'a'
+const ၡ = 6;
+console.log(ၡ ** ၡ);
+// lower 8 bits aren't a valid unicode character
+const ဒ = 7;
+console.log(ဒ ** ဒ);
+// a mix, for good measure
+const ဒၡ𝑀 = 7;
+console.log(ဒၡ𝑀 ** ဒၡ𝑀);
+const ၡ𝑀ဒ = 7;
+console.log(ၡ𝑀ဒ ** ၡ𝑀ဒ);
+const 𝑀ဒၡ = 7;
+console.log(𝑀ဒၡ ** 𝑀ဒၡ);
+const 𝓱𝓮𝓵𝓵𝓸 = "𝔀𝓸𝓻𝓵𝓭";
+const Ɐⱱ = "ok"; // BMP
+const 𓀸𓀹𓀺 = "ok"; // SMP
+const 𡚭𡚮𡚯 = "ok"; // SIP
+const 𡚭𓀺ⱱ𝓮 = "ok";
+const 𓀺ⱱ𝓮𡚭 = "ok";
+const ⱱ𝓮𡚭𓀺 = "ok";
+const 𝓮𡚭𓀺ⱱ = "ok";
diff --git a/tests/baselines/reference/extendedUnicodePlaneIdentifiers.symbols b/tests/baselines/reference/extendedUnicodePlaneIdentifiers.symbols
index 9d78bfaee05..170b82da7e9 100644
--- a/tests/baselines/reference/extendedUnicodePlaneIdentifiers.symbols
+++ b/tests/baselines/reference/extendedUnicodePlaneIdentifiers.symbols
@@ -12,3 +12,80 @@ console.log(𝑀 + 𝑚); // 9
 >𝑀 : Symbol(𝑀, Decl(extendedUnicodePlaneIdentifiers.ts, 1, 5))
 >𝑚 : Symbol(𝑚, Decl(extendedUnicodePlaneIdentifiers.ts, 0, 5))
 
+// lower 8 bits look like 'a'
+const ၡ = 6;
+>ၡ : Symbol(ၡ, Decl(extendedUnicodePlaneIdentifiers.ts, 5, 5))
+
+console.log(ၡ ** ၡ);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>ၡ : Symbol(ၡ, Decl(extendedUnicodePlaneIdentifiers.ts, 5, 5))
+>ၡ : Symbol(ၡ, Decl(extendedUnicodePlaneIdentifiers.ts, 5, 5))
+
+// lower 8 bits aren't a valid unicode character
+const ဒ = 7;
+>ဒ : Symbol(ဒ, Decl(extendedUnicodePlaneIdentifiers.ts, 9, 5))
+
+console.log(ဒ ** ဒ);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>ဒ : Symbol(ဒ, Decl(extendedUnicodePlaneIdentifiers.ts, 9, 5))
+>ဒ : Symbol(ဒ, Decl(extendedUnicodePlaneIdentifiers.ts, 9, 5))
+
+// a mix, for good measure
+const ဒၡ𝑀 = 7;
+>ဒၡ𝑀 : Symbol(ဒၡ𝑀, Decl(extendedUnicodePlaneIdentifiers.ts, 13, 5))
+
+console.log(ဒၡ𝑀 ** ဒၡ𝑀);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>ဒၡ𝑀 : Symbol(ဒၡ𝑀, Decl(extendedUnicodePlaneIdentifiers.ts, 13, 5))
+>ဒၡ𝑀 : Symbol(ဒၡ𝑀, Decl(extendedUnicodePlaneIdentifiers.ts, 13, 5))
+
+const ၡ𝑀ဒ = 7;
+>ၡ𝑀ဒ : Symbol(ၡ𝑀ဒ, Decl(extendedUnicodePlaneIdentifiers.ts, 16, 5))
+
+console.log(ၡ𝑀ဒ ** ၡ𝑀ဒ);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>ၡ𝑀ဒ : Symbol(ၡ𝑀ဒ, Decl(extendedUnicodePlaneIdentifiers.ts, 16, 5))
+>ၡ𝑀ဒ : Symbol(ၡ𝑀ဒ, Decl(extendedUnicodePlaneIdentifiers.ts, 16, 5))
+
+const 𝑀ဒၡ = 7;
+>𝑀ဒၡ : Symbol(𝑀ဒၡ, Decl(extendedUnicodePlaneIdentifiers.ts, 19, 5))
+
+console.log(𝑀ဒၡ ** 𝑀ဒၡ);
+>console.log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>console : Symbol(console, Decl(lib.dom.d.ts, --, --))
+>log : Symbol(Console.log, Decl(lib.dom.d.ts, --, --))
+>𝑀ဒၡ : Symbol(𝑀ဒၡ, Decl(extendedUnicodePlaneIdentifiers.ts, 19, 5))
+>𝑀ဒၡ : Symbol(𝑀ဒၡ, Decl(extendedUnicodePlaneIdentifiers.ts, 19, 5))
+
+const 𝓱𝓮𝓵𝓵𝓸 = "𝔀𝓸𝓻𝓵𝓭";
+>𝓱𝓮𝓵𝓵𝓸 : Symbol(𝓱𝓮𝓵𝓵𝓸, Decl(extendedUnicodePlaneIdentifiers.ts, 22, 5))
+
+const Ɐⱱ = "ok"; // BMP
+>Ɐⱱ : Symbol(Ɐⱱ, Decl(extendedUnicodePlaneIdentifiers.ts, 24, 5))
+
+const 𓀸𓀹𓀺 = "ok"; // SMP
+>𓀸𓀹𓀺 : Symbol(𓀸𓀹𓀺, Decl(extendedUnicodePlaneIdentifiers.ts, 26, 5))
+
+const 𡚭𡚮𡚯 = "ok"; // SIP
+>𡚭𡚮𡚯 : Symbol(𡚭𡚮𡚯, Decl(extendedUnicodePlaneIdentifiers.ts, 28, 5))
+
+const 𡚭𓀺ⱱ𝓮 = "ok";
+>𡚭𓀺ⱱ𝓮 : Symbol(𡚭𓀺ⱱ𝓮, Decl(extendedUnicodePlaneIdentifiers.ts, 30, 5))
+
+const 𓀺ⱱ𝓮𡚭 = "ok";
+>𓀺ⱱ𝓮𡚭 : Symbol(𓀺ⱱ𝓮𡚭, Decl(extendedUnicodePlaneIdentifiers.ts, 32, 5))
+
+const ⱱ𝓮𡚭𓀺 = "ok";
+>ⱱ𝓮𡚭𓀺 : Symbol(ⱱ𝓮𡚭𓀺, Decl(extendedUnicodePlaneIdentifiers.ts, 34, 5))
+
+const 𝓮𡚭𓀺ⱱ = "ok";
+>𝓮𡚭𓀺ⱱ : Symbol(𝓮𡚭𓀺ⱱ, Decl(extendedUnicodePlaneIdentifiers.ts, 36, 5))
+
diff --git a/tests/baselines/reference/extendedUnicodePlaneIdentifiers.types b/tests/baselines/reference/extendedUnicodePlaneIdentifiers.types
index 5de90b68b6d..4ee200dd5d3 100644
--- a/tests/baselines/reference/extendedUnicodePlaneIdentifiers.types
+++ b/tests/baselines/reference/extendedUnicodePlaneIdentifiers.types
@@ -16,3 +16,103 @@ console.log(𝑀 + 𝑚); // 9
 >𝑀 : 5
 >𝑚 : 4
 
+// lower 8 bits look like 'a'
+const ၡ = 6;
+>ၡ : 6
+>6 : 6
+
+console.log(ၡ ** ၡ);
+>console.log(ၡ ** ၡ) : void
+>console.log : (message?: any, ...optionalParams: any[]) => void
+>console : Console
+>log : (message?: any, ...optionalParams: any[]) => void
+>ၡ ** ၡ : number
+>ၡ : 6
+>ၡ : 6
+
+// lower 8 bits aren't a valid unicode character
+const ဒ = 7;
+>ဒ : 7
+>7 : 7
+
+console.log(ဒ ** ဒ);
+>console.log(ဒ ** ဒ) : void
+>console.log : (message?: any, ...optionalParams: any[]) => void
+>console : Console
+>log : (message?: any, ...optionalParams: any[]) => void
+>ဒ ** ဒ : number
+>ဒ : 7
+>ဒ : 7
+
+// a mix, for good measure
+const ဒၡ𝑀 = 7;
+>ဒၡ𝑀 : 7
+>7 : 7
+
+console.log(ဒၡ𝑀 ** ဒၡ𝑀);
+>console.log(ဒၡ𝑀 ** ဒၡ𝑀) : void
+>console.log : (message?: any, ...optionalParams: any[]) => void
+>console : Console
+>log : (message?: any, ...optionalParams: any[]) => void
+>ဒၡ𝑀 ** ဒၡ𝑀 : number
+>ဒၡ𝑀 : 7
+>ဒၡ𝑀 : 7
+
+const ၡ𝑀ဒ = 7;
+>ၡ𝑀ဒ : 7
+>7 : 7
+
+console.log(ၡ𝑀ဒ ** ၡ𝑀ဒ);
+>console.log(ၡ𝑀ဒ ** ၡ𝑀ဒ) : void
+>console.log : (message?: any, ...optionalParams: any[]) => void
+>console : Console
+>log : (message?: any, ...optionalParams: any[]) => void
+>ၡ𝑀ဒ ** ၡ𝑀ဒ : number
+>ၡ𝑀ဒ : 7
+>ၡ𝑀ဒ : 7
+
+const 𝑀ဒၡ = 7;
+>𝑀ဒၡ : 7
+>7 : 7
+
+console.log(𝑀ဒၡ ** 𝑀ဒၡ);
+>console.log(𝑀ဒၡ ** 𝑀ဒၡ) : void
+>console.log : (message?: any, ...optionalParams: any[]) => void
+>console : Console
+>log : (message?: any, ...optionalParams: any[]) => void
+>𝑀ဒၡ ** 𝑀ဒၡ : number
+>𝑀ဒၡ : 7
+>𝑀ဒၡ : 7
+
+const 𝓱𝓮𝓵𝓵𝓸 = "𝔀𝓸𝓻𝓵𝓭";
+>𝓱𝓮𝓵𝓵𝓸 : "𝔀𝓸𝓻𝓵𝓭"
+>"𝔀𝓸𝓻𝓵𝓭" : "𝔀𝓸𝓻𝓵𝓭"
+
+const Ɐⱱ = "ok"; // BMP
+>Ɐⱱ : "ok"
+>"ok" : "ok"
+
+const 𓀸𓀹𓀺 = "ok"; // SMP
+>𓀸𓀹𓀺 : "ok"
+>"ok" : "ok"
+
+const 𡚭𡚮𡚯 = "ok"; // SIP
+>𡚭𡚮𡚯 : "ok"
+>"ok" : "ok"
+
+const 𡚭𓀺ⱱ𝓮 = "ok";
+>𡚭𓀺ⱱ𝓮 : "ok"
+>"ok" : "ok"
+
+const 𓀺ⱱ𝓮𡚭 = "ok";
+>𓀺ⱱ𝓮𡚭 : "ok"
+>"ok" : "ok"
+
+const ⱱ𝓮𡚭𓀺 = "ok";
+>ⱱ𝓮𡚭𓀺 : "ok"
+>"ok" : "ok"
+
+const 𝓮𡚭𓀺ⱱ = "ok";
+>𝓮𡚭𓀺ⱱ : "ok"
+>"ok" : "ok"
+
diff --git a/tests/cases/compiler/extendedUnicodePlaneIdentifiers.ts b/tests/cases/compiler/extendedUnicodePlaneIdentifiers.ts
index f207571549c..9210ee3b7c5 100644
--- a/tests/cases/compiler/extendedUnicodePlaneIdentifiers.ts
+++ b/tests/cases/compiler/extendedUnicodePlaneIdentifiers.ts
@@ -2,3 +2,37 @@
 const 𝑚 = 4;
 const 𝑀 = 5;
 console.log(𝑀 + 𝑚); // 9
+
+// lower 8 bits look like 'a'
+const ၡ = 6;
+console.log(ၡ ** ၡ);
+
+// lower 8 bits aren't a valid unicode character
+const ဒ = 7;
+console.log(ဒ ** ဒ);
+
+// a mix, for good measure
+const ဒၡ𝑀 = 7;
+console.log(ဒၡ𝑀 ** ဒၡ𝑀);
+
+const ၡ𝑀ဒ = 7;
+console.log(ၡ𝑀ဒ ** ၡ𝑀ဒ);
+
+const 𝑀ဒၡ = 7;
+console.log(𝑀ဒၡ ** 𝑀ဒၡ);
+
+const 𝓱𝓮𝓵𝓵𝓸 = "𝔀𝓸𝓻𝓵𝓭";
+
+const Ɐⱱ = "ok"; // BMP
+
+const 𓀸𓀹𓀺 = "ok"; // SMP
+
+const 𡚭𡚮𡚯 = "ok"; // SIP
+
+const 𡚭𓀺ⱱ𝓮 = "ok";
+
+const 𓀺ⱱ𝓮𡚭 = "ok";
+
+const ⱱ𝓮𡚭𓀺 = "ok";
+
+const 𝓮𡚭𓀺ⱱ = "ok";