diff --git a/stix2/base.py b/stix2/base.py
index b0cf6ff..b26f044 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -40,14 +40,7 @@ class _STIXBase(collections.Mapping):
     """Base class for STIX object types"""
 
     def object_properties(self):
-        props = set(self._properties.keys())
-        custom_props = list(set(self._inner.keys()) - props)
-        custom_props.sort()
-
-        all_properties = list(self._properties.keys())
-        all_properties.extend(custom_props)  # Any custom properties to the bottom
-
-        return all_properties
+        return list(self._properties.keys())
 
     def _check_property(self, prop_name, prop, kwargs):
         if prop_name not in kwargs:
@@ -109,6 +102,13 @@ class _STIXBase(collections.Mapping):
             extra_kwargs = list(set(kwargs) - set(cls._properties))
             if extra_kwargs:
                 raise ExtraPropertiesError(cls, extra_kwargs)
+        else:
+            from .properties import CustomProperty
+
+            # The custom properties will get added to the bottom.
+            # Matched with a CustomProperty.
+            extra_kwargs = list(set(kwargs) - set(cls._properties))
+            self._properties.update([(x, CustomProperty()) for x in extra_kwargs])
 
         # Remove any keyword arguments whose value is None
         setting_kwargs = {}
diff --git a/stix2/properties.py b/stix2/properties.py
index ca7f04c..5b480ac 100644
--- a/stix2/properties.py
+++ b/stix2/properties.py
@@ -393,3 +393,15 @@ class PatternProperty(StringProperty):
             raise ValueError(str(errors[0]))
 
         return self.string_type(value)
+
+
+class CustomProperty(Property):
+    """
+    The custom property class can be used as a base to extend further
+    functionality of a custom property.
+
+    Note:
+        This class is used internally to signal the use of any custom property
+        that is parsed by any object or `parse()` method and allow_custom=True.
+    """
+    pass