diff --git a/lib/output.js b/lib/output.js
index 8ba98b69..91f7c504 100644
--- a/lib/output.js
+++ b/lib/output.js
@@ -67,7 +67,8 @@ function OutputStream(options) {
         screw_ie8        : false,
         preamble         : null,
         quote_style      : 0,
-        keep_quoted_props: false
+        keep_quoted_props: false,
+        ecma             : 5,
     }, true);
 
     var indentation = 0;
@@ -80,7 +81,11 @@ function OutputStream(options) {
         return str.replace(/[\ud800-\udbff][\udc00-\udfff]|[\u0000-\u001f\u007f-\uffff]/g, function(ch) {
             var code = get_full_char_code(ch, 0).toString(16);
 
-            if ((identifier && code.length === 1) || code.length > 4) {
+            if ((identifier && code.length === 1 && !options.es5) || code.length > 4) {
+                if (options.ecma < 6) {
+                    return "\\u" + ch.charCodeAt(0).toString(16) + "\\u"
+                        + ch.charCodeAt(1).toString(16);
+                }
                 return "\\u{" + code + "}";
             } else if (code.length <= 2 && !identifier) {
                 while (code.length < 2) code = "0" + code;
diff --git a/test/compress/unicode.js b/test/compress/unicode.js
index 9651c6df..f08072e0 100644
--- a/test/compress/unicode.js
+++ b/test/compress/unicode.js
@@ -17,6 +17,7 @@ unicode_parse_variables: {
 }
 
 unicode_escaped_identifier: {
+    beautify = {ecma: 6}
     input: {
         var \u{61} = "foo";
         var \u{10000} = "bar";
@@ -25,7 +26,7 @@ unicode_escaped_identifier: {
 }
 
 unicode_identifier_ascii_only: {
-    beautify = {ascii_only: true}
+    beautify = {ascii_only: true, ecma: 6}
     input: {
         var \u{0061} = "hi";
         var bar = "h\u{0065}llo";
@@ -35,9 +36,67 @@ unicode_identifier_ascii_only: {
 }
 
 unicode_string_literals: {
-    beautify = {ascii_only: true}
+    beautify = {ascii_only: true, ecma: 6}
     input: {
         var a = "6 length unicode character: \u{101111}";
     }
     expect_exact: 'var a="6 length unicode character: \\u{101111}";'
 }
+
+unicode_output_es5_surrogates: {
+    beautify = {ascii_only: true, ecma: 5}
+    input: {
+        var \u{10000} = "6 length unicode character: \u{10FFFF}";
+    }
+    expect_exact: 'var \\ud800\\udc00="6 length unicode character: \\udbff\\udfff";'
+}
+
+check_escape_style: {
+    beautify = {ascii_only: true, ecma: 6}
+    input: {
+        var a = "\x01";
+        var \ua0081 = "\x10"; // \u0081 only in ID_Continue
+        var \u0100 = "\u0100";
+        var \u1000 = "\u1000";
+        var \u{10000} = "\u{10000}";
+        var \u{2f800} = "\u{100000}";
+    }
+    expect_exact: 'var a="\\x01";var \\ua0081="\\x10";var \\u0100="\\u0100";var \\u1000="\\u1000";var \\u{10000}="\\u{10000}";var \\u{2f800}="\\u{100000}";'
+}
+
+check_escape_style_es5: {
+    beautify = {ascii_only: true, ecma: 5}
+    input: {
+        var a = "\x01";
+        var \ua0081 = "\x10"; // \u0081 only in ID_Continue
+        var \u0100 = "\u0100";
+        var \u1000 = "\u1000";
+        var \u{10000} = "\u{10000}";
+        var \u{2f800} = "\u{100000}";
+    }
+    expect_exact: 'var a="\\x01";var \\ua0081="\\x10";var \\u0100="\\u0100";var \\u1000="\\u1000";var \\ud800\\udc00="\\ud800\\udc00";var \\ud87e\\udc00="\\udbc0\\udc00";'
+}
+
+ID_continue_with_surrogate_pair: {
+    beautify = {ascii_only: true, ecma: 6}
+    input: {
+        var \u{2f800}\u{2f800}\u{2f800}\u{2f800} = "\u{100000}\u{100000}\u{100000}\u{100000}\u{100000}";
+    }
+    expect_exact: 'var \\u{2f800}\\u{2f800}\\u{2f800}\\u{2f800}="\\u{100000}\\u{100000}\\u{100000}\\u{100000}\\u{100000}";'
+}
+
+escape_non_escaped_identifier: {
+    beautify = {ascii_only: true, ecma: 6}
+    input: {
+        var µþ = "µþ";
+    }
+    expect_exact: 'var \\u00b5\\u00fe="\\xb5\\xfe";'
+}
+
+non_escape_2_non_escape: {
+    beautify = {ascii_only: false, ecma: 6}
+    input: {
+        var µþ = "µþ";
+    }
+    expect_exact: 'var µþ="µþ";'
+}
\ No newline at end of file
diff --git a/test/mocha/unicode.js b/test/mocha/unicode.js
index a7cc12dd..d23b4a46 100644
--- a/test/mocha/unicode.js
+++ b/test/mocha/unicode.js
@@ -126,7 +126,7 @@ describe("Unicode", function() {
 
         for (var i = 0; i < tests.length; i++) {
             assert.strictEqual(uglify.minify(tests[i][0], {
-                fromString: true, output: { ascii_only: true}
+                fromString: true, output: { ascii_only: true, ecma: 6}
             }).code, tests[i][1]);
         }
     });