diff --git a/swh/core/api/asynchronous.py b/swh/core/api/asynchronous.py
--- a/swh/core/api/asynchronous.py
+++ b/swh/core/api/asynchronous.py
@@ -27,7 +27,7 @@
     if content_type == 'application/x-msgpack':
         r = msgpack_loads(data)
     elif content_type == 'application/json':
-        r = json.loads(data, cls=SWHJSONDecoder)
+        r = json.loads(data.decode(), cls=SWHJSONDecoder)
     else:
         raise ValueError('Wrong content type `%s` for API request'
                          % content_type)
diff --git a/swh/core/api/tests/test_async.py b/swh/core/api/tests/test_async.py
new file mode 100644
--- /dev/null
+++ b/swh/core/api/tests/test_async.py
@@ -0,0 +1,114 @@
+# Copyright (C) 2019  The Software Heritage developers
+# See the AUTHORS file at the top-level directory of this distribution
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+import datetime
+import json
+
+import msgpack
+
+import pytest
+
+from swh.core.api.asynchronous import RPCServerApp
+from swh.core.api.asynchronous import encode_data_server, decode_request
+from swh.core.api.serializers import msgpack_dumps, SWHJSONEncoder
+
+
+pytest_plugins = ['aiohttp.pytest_plugin', 'pytester']
+
+
+async def root(request):
+    return encode_data_server('toor')
+
+STRUCT = {'txt': 'something stupid',
+          # 'date': datetime.date(2019, 6, 9),  # not supported
+          'datetime': datetime.datetime(2019, 6, 9, 10, 12),
+          'timedelta': datetime.timedelta(days=-2, hours=3),
+          'int': 42,
+          'float': 3.14,
+          'subdata': {'int': 42,
+                      'datetime': datetime.datetime(2019, 6, 10, 11, 12),
+                      },
+          'list': [42, datetime.datetime(2019, 9, 10, 11, 12), 'ok'],
+          }
+
+
+async def struct(request):
+    return encode_data_server(STRUCT)
+
+
+async def echo(request):
+    data = await decode_request(request)
+    return encode_data_server(data)
+
+
+@pytest.fixture
+def app():
+    app = RPCServerApp()
+    app.router.add_route('GET', '/', root)
+    app.router.add_route('GET', '/struct', struct)
+    app.router.add_route('POST', '/echo', echo)
+    return app
+
+
+async def test_get_simple(app,  aiohttp_client) -> None:
+    assert app is not None
+
+    cli = await aiohttp_client(app)
+    resp = await cli.get('/')
+    assert resp.status == 200
+    data = await resp.read()
+    value = msgpack.unpackb(data, raw=False)
+    assert value == 'toor'
+
+
+async def test_get_struct(app,  aiohttp_client) -> None:
+    """Test returned structured from a simple GET data is OK"""
+    cli = await aiohttp_client(app)
+    resp = await cli.get('/struct')
+    assert resp.status == 200
+    assert resp.headers['Content-Type'] == 'application/x-msgpack'
+    assert (await decode_request(resp)) == STRUCT
+
+
+async def test_post_struct_msgpack(app,  aiohttp_client) -> None:
+    """Test that msgpack encoded posted struct data is returned as is"""
+    cli = await aiohttp_client(app)
+    # simple struct
+    resp = await cli.post(
+        '/echo',
+        headers={'Content-Type': 'application/x-msgpack'},
+        data=msgpack_dumps({'toto': 42}))
+    assert resp.status == 200
+    assert resp.headers['Content-Type'] == 'application/x-msgpack'
+    assert (await decode_request(resp)) == {'toto': 42}
+    # complex struct
+    resp = await cli.post(
+        '/echo',
+        headers={'Content-Type': 'application/x-msgpack'},
+        data=msgpack_dumps(STRUCT))
+    assert resp.status == 200
+    assert resp.headers['Content-Type'] == 'application/x-msgpack'
+    assert (await decode_request(resp)) == STRUCT
+
+
+async def test_post_struct_json(app,  aiohttp_client) -> None:
+    """Test that json encoded posted struct data is returned as is"""
+    cli = await aiohttp_client(app)
+
+    resp = await cli.post(
+        '/echo',
+        headers={'Content-Type': 'application/json'},
+        data=json.dumps({'toto': 42}, cls=SWHJSONEncoder))
+    assert resp.status == 200
+    assert resp.headers['Content-Type'] == 'application/x-msgpack'
+    assert (await decode_request(resp)) == {'toto': 42}
+
+    resp = await cli.post(
+        '/echo',
+        headers={'Content-Type': 'application/json'},
+        data=json.dumps(STRUCT, cls=SWHJSONEncoder))
+    assert resp.status == 200
+    assert resp.headers['Content-Type'] == 'application/x-msgpack'
+    assert (await decode_request(resp)) == STRUCT