diff --git a/flask/json.py b/flask/json.py

index 45ba3240..1ee0586e 100644

--- a/flask/json.py

+++ b/flask/json.py

@@ -8,6 +8,7 @@

     :copyright: (c) 2012 by Armin Ronacher.

     :license: BSD, see LICENSE for more details.

 """

+import codecs

 import io

 import uuid

 from datetime import datetime

@@ -111,6 +112,49 @@ def _load_arg_defaults(kwargs):

         kwargs.setdefault('cls', JSONDecoder)

+def detect_encoding(data):

+    """Detect which UTF codec was used to encode the given bytes.

+

+    The latest JSON standard (:rfc:`8259`) suggests that only UTF-8 is

+    accepted. Older documents allowed 8, 16, or 32. 16 and 32 can be big

+    or little endian. Some editors or libraries may prepend a BOM.

+

+    :param data: Bytes in unknown UTF encoding.

+    :return: UTF encoding name

+    """

+    head = data[:4]

+

+    if head[:3] == codecs.BOM_UTF8:

+        return 'utf-8-sig'

+

+    if b'\x00' not in head:

+        return 'utf-8'

+

+    if head in (codecs.BOM_UTF32_BE, codecs.BOM_UTF32_LE):

+        return 'utf-32'

+

+    if head[:2] in (codecs.BOM_UTF16_BE, codecs.BOM_UTF16_LE):

+        return 'utf-16'

+

+    if len(head) == 4:

+        if head[:3] == b'\x00\x00\x00':

+            return 'utf-32-be'

+

+        if head[::2] == b'\x00\x00':

+            return 'utf-16-be'

+

+        if head[1:] == b'\x00\x00\x00':

+            return 'utf-32-le'

+

+        if head[1::2] == b'\x00\x00':

+            return 'utf-16-le'

+

+    if len(head) == 2:

+        return 'utf-16-be' if head.startswith(b'\x00') else 'utf-16-le'

+

+    return 'utf-8'

+

+

 def dumps(obj, **kwargs):

     """Serialize ``obj`` to a JSON formatted ``str`` by using the application's

     configured encoder (:attr:`~flask.Flask.json_encoder`) if there is an

@@ -145,7 +189,10 @@ def loads(s, **kwargs):

     """

     _load_arg_defaults(kwargs)

     if isinstance(s, bytes):

-        s = s.decode(kwargs.pop('encoding', None) or 'utf-8')

+        encoding = kwargs.pop('encoding', None)

+        if encoding is None:

+            encoding = detect_encoding(s)

+        s = s.decode(encoding)

     return _json.loads(s, **kwargs)

diff --git a/flask/testsuite/helpers.py b/flask/testsuite/helpers.py

index 636f67fa..12e10d96 100644

--- a/flask/testsuite/helpers.py

+++ b/flask/testsuite/helpers.py

@@ -15,6 +15,8 @@ import unittest

 from logging import StreamHandler

 from flask.testsuite import FlaskTestCase, catch_warnings, catch_stderr

 from werkzeug.http import parse_cache_control_header, parse_options_header

+

+from flask import json

 from flask._compat import StringIO, text_type

@@ -29,6 +31,16 @@ def has_encoding(name):

 class JSONTestCase(FlaskTestCase):

+    def test_detect_encoding(self):

+        values = (1, 't', True, False, None, [], [1,2,3], {}, {'foo': u'🐍'},)

+        encodings = ('utf-8', 'utf-8-sig', 'utf-16-le', 'utf-16-be', 'utf-16', 'utf-32-le', 'utf-32-be', 'utf-32',)

+

+        for encoding in encodings:

+            for value in values:

+                data = json.dumps(value).encode(encoding)

+                self.assert_equal(json.detect_encoding(data), encoding)

+                self.assert_equal(json.loads(data),value)

+

     def test_json_bad_requests(self):

         app = flask.Flask(__name__)

         @app.route('/json', methods=['POST'])

@@ -38,18 +50,6 @@ class JSONTestCase(FlaskTestCase):

         rv = c.post('/json', data='malformed', content_type='application/json')

         self.assert_equal(rv.status_code, 400)

-    def test_json_body_encoding(self):

-        app = flask.Flask(__name__)

-        app.testing = True

-        @app.route('/')

-        def index():

-            return flask.request.get_json()

-

-        c = app.test_client()

-        resp = c.get('/', data=u'"Hällo Wörld"'.encode('iso-8859-15'),

-                     content_type='application/json; charset=iso-8859-15')

-        self.assert_equal(resp.data, u'Hällo Wörld'.encode('utf-8'))

-

     def test_jsonify(self):

         d = dict(a=23, b=42, c=[1, 2, 3])

         app = flask.Flask(__name__)

diff --git a/flask/wrappers.py b/flask/wrappers.py

index 1a17824a..0d6f068d 100644

--- a/flask/wrappers.py

+++ b/flask/wrappers.py

@@ -127,17 +127,10 @@ class Request(RequestBase):

         if self.mimetype != 'application/json' and not force:

             return None

-        # We accept a request charset against the specification as

-        # certain clients have been using this in the past.  This

-        # fits our general approach of being nice in what we accept

-        # and strict in what we send out.

-        request_charset = self.mimetype_params.get('charset')

+        data = _get_data(self, cache)

+

         try:

-            data = _get_data(self, cache)

-            if request_charset is not None:

-                rv = json.loads(data, encoding=request_charset)

-            else:

-                rv = json.loads(data)

+            rv = json.loads(data)

         except ValueError as e:

             if silent:

                 rv = None