diff --git a/odml/property.py b/odml/property.py
index e6fb3a5d..d3ea8b8d 100644
--- a/odml/property.py
+++ b/odml/property.py
@@ -329,6 +329,14 @@ def uncertainty(self):
     def uncertainty(self, new_value):
         if new_value == "":
             new_value = None
+
+        if new_value and not isinstance(new_value, (int, float)):
+            try:
+                new_value = float(new_value)
+            except ValueError:
+                raise ValueError("odml.Property.uncertainty: passed uncertainty '%s' "
+                                 "is not float or int." % new_value)
+
         self._uncertainty = new_value
 
     @property
diff --git a/test/test_property.py b/test/test_property.py
index 58285a5e..0877e3b3 100644
--- a/test/test_property.py
+++ b/test/test_property.py
@@ -14,7 +14,7 @@ def test_simple_attributes(self):
         p_name = "propertyName"
         p_origin = "from over there"
         p_unit = "pears"
-        p_uncertainty = "+-12"
+        p_uncertainty = "12"
         p_ref = "4 8 15 16 23"
         p_def = "an odml test property"
         p_dep = "yes"
@@ -40,8 +40,8 @@ def test_simple_attributes(self):
         self.assertEqual(prop.value_origin, "%s_edit" % p_origin)
         prop.unit = "%s_edit" % p_unit
         self.assertEqual(prop.unit, "%s_edit" % p_unit)
-        prop.uncertainty = "%s_edit" % p_uncertainty
-        self.assertEqual(prop.uncertainty, "%s_edit" % p_uncertainty)
+        prop.uncertainty = 13
+        self.assertEqual(prop.uncertainty, 13.0)
         prop.reference = "%s_edit" % p_ref
         self.assertEqual(prop.reference, "%s_edit" % p_ref)
         prop.definition = "%s_edit" % p_def
diff --git a/test/test_rdf_writer.py b/test/test_rdf_writer.py
index 9666a01b..0d1d30a6 100644
--- a/test/test_rdf_writer.py
+++ b/test/test_rdf_writer.py
@@ -159,7 +159,7 @@ def test_adding_other_entities_properties(self):
         p_unit = "u1"
         p_name = "p1"
         p_def = "p comment"
-        p_uncertainty = "un"
+        p_uncertainty = 13.0
         p_dtype = "string"
         p_value_origin = "value"
         p_ref = "p_ref"