diff --git a/stix2/custom.py b/stix2/custom.py
index a00498b..81991e9 100644
--- a/stix2/custom.py
+++ b/stix2/custom.py
@@ -53,7 +53,7 @@ def _custom_marking_builder(cls, type, properties, version):
     return _CustomMarking
 
 
-def _custom_observable_builder(cls, type, properties, version):
+def _custom_observable_builder(cls, type, properties, version, id_contrib_props=[]):
     class _CustomObservable(cls, _Observable):
 
         if not re.match(TYPE_REGEX, type):
@@ -98,6 +98,8 @@ def _custom_observable_builder(cls, type, properties, version):
 
         _type = type
         _properties = OrderedDict(properties)
+        if version != '2.0':
+            _id_contributing_properties = id_contrib_props
 
         def __init__(self, **kwargs):
             _Observable.__init__(self, **kwargs)
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index ed560a6..5c23362 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -966,7 +966,7 @@ class X509Certificate(_Observable):
         self._check_at_least_one_property(att_list)
 
 
-def CustomObservable(type='x-custom-observable', properties=None):
+def CustomObservable(type='x-custom-observable', properties=None, id_contrib_props=[]):
     """Custom STIX Cyber Observable Object type decorator.
 
     Example:
@@ -987,7 +987,7 @@ def CustomObservable(type='x-custom-observable', properties=None):
             properties,
             [('extensions', ExtensionsProperty(spec_version='2.1', enclosing_type=type))],
         ]))
-        return _custom_observable_builder(cls, type, _properties, '2.1')
+        return _custom_observable_builder(cls, type, _properties, '2.1', id_contrib_props)
     return wrapper