diff --git a/qcodes/data/gnuplot_format.py b/qcodes/data/gnuplot_format.py
index decaa7d3574..28052fd797c 100644
--- a/qcodes/data/gnuplot_format.py
+++ b/qcodes/data/gnuplot_format.py
@@ -3,7 +3,7 @@
 import math
 import json
 
-from qcodes.utils.helpers import deep_update
+from qcodes.utils.helpers import deep_update, NumpyJSONEncoder
 from .data_array import DataArray
 from .format import Formatter
 
@@ -297,7 +297,7 @@ def write_metadata(self, data_set, read_first=True):
         fn = io_manager.join(location, self.metadata_file)
         with io_manager.open(fn, 'w', encoding='utf8') as snap_file:
             json.dump(data_set.metadata, snap_file, sort_keys=True,
-                      indent=4, ensure_ascii=False)
+                      indent=4, ensure_ascii=False, cls=NumpyJSONEncoder)
 
     def read_metadata(self, data_set):
         io_manager = data_set.io
diff --git a/qcodes/tests/test_json.py b/qcodes/tests/test_json.py
new file mode 100644
index 00000000000..0c81e96ebc4
--- /dev/null
+++ b/qcodes/tests/test_json.py
@@ -0,0 +1,44 @@
+from unittest import TestCase
+import numpy as np
+import json
+
+from qcodes.utils.helpers import NumpyJSONEncoder
+
+
+class TestNumpyJson(TestCase):
+
+    def setUp(self):
+        self.metadata = {
+            'name': 'Rapunzel',
+            'age': np.int64(12),
+            'height': np.float64(112.234),
+            'scores': np.linspace(0, 42, num=3),
+            # include some regular values to ensure they work right
+            # with our encoder
+            'weight': 19,
+            'length': 45.23,
+            'points': [12, 24, 48]
+        }
+
+    def test_numpy_fail(self):
+        metadata = self.metadata
+        with self.assertRaises(TypeError):
+            json.dumps(metadata, sort_keys=True, indent=4, ensure_ascii=False)
+
+    def test_numpy_good(self):
+        metadata = self.metadata
+        data = json.dumps(metadata, sort_keys=True, indent=4,
+                          ensure_ascii=False, cls=NumpyJSONEncoder)
+        data_dict = json.loads(data)
+
+        metadata = {
+            'name': 'Rapunzel',
+            'age': 12,
+            'height': 112.234,
+            'scores': [0, 21, 42],
+            'weight': 19,
+            'length': 45.23,
+            'points': [12, 24, 48]
+        }
+
+        self.assertEqual(metadata, data_dict)
diff --git a/qcodes/utils/helpers.py b/qcodes/utils/helpers.py
index b00bc368a99..343f2eec63e 100644
--- a/qcodes/utils/helpers.py
+++ b/qcodes/utils/helpers.py
@@ -6,10 +6,25 @@
 import sys
 import io
 import numpy as np
+import json
 
 _tprint_times = {}
 
 
+class NumpyJSONEncoder(json.JSONEncoder):
+    """Return numpy types as standard types."""
+    # http://stackoverflow.com/questions/27050108/convert-numpy-type-to-python
+    # http://stackoverflow.com/questions/9452775/converting-numpy-dtypes-to-native-python-types/11389998#11389998
+    def default(self, obj):
+        if isinstance(obj, np.integer):
+            return int(obj)
+        elif isinstance(obj, np.floating):
+            return float(obj)
+        elif isinstance(obj, np.ndarray):
+            return obj.tolist()
+        else:
+            return super(NumpyJSONEncoder, self).default(obj)
+
 def tprint(string, dt=1, tag='default'):
     """ Print progress of a loop every dt seconds """
     ptime = _tprint_times.get(tag, 0)