diff --git a/docs/guide/custom.ipynb b/docs/guide/custom.ipynb
index bc19749..e651084 100644
--- a/docs/guide/custom.ipynb
+++ b/docs/guide/custom.ipynb
@@ -1155,10 +1155,10 @@
    "source": [
     "from stix2 import File, CustomExtension\n",
     "\n",
-    "@CustomExtension(File, 'x-new-ext', {\n",
-    "    'property1': properties.StringProperty(required=True),\n",
-    "    'property2': properties.IntegerProperty(),\n",
-    "})\n",
+    "@CustomExtension(File, 'x-new-ext', [\n",
+    "    ('property1', properties.StringProperty(required=True)),\n",
+    "    ('property2', properties.IntegerProperty()),\n",
+    "])\n",
     "class NewExtension():\n",
     "    pass\n",
     "\n",
diff --git a/stix2/test/test_custom.py b/stix2/test/test_custom.py
index cc8b32b..a14503f 100644
--- a/stix2/test/test_custom.py
+++ b/stix2/test/test_custom.py
@@ -435,10 +435,10 @@ def test_observed_data_with_custom_observable_object():
     assert ob_data.objects['0'].property1 == 'something'
 
 
-@stix2.observables.CustomExtension(stix2.DomainName, 'x-new-ext', {
-    'property1': stix2.properties.StringProperty(required=True),
-    'property2': stix2.properties.IntegerProperty(),
-})
+@stix2.observables.CustomExtension(stix2.DomainName, 'x-new-ext', [
+    ('property1', stix2.properties.StringProperty(required=True)),
+    ('property2', stix2.properties.IntegerProperty()),
+])
 class NewExtension():
     def __init__(self, property2=None, **kwargs):
         if property2 and property2 < 10:
@@ -485,9 +485,9 @@ def test_custom_extension_invalid_observable():
     class Foo(object):
         pass
     with pytest.raises(ValueError) as excinfo:
-        @stix2.observables.CustomExtension(Foo, 'x-new-ext', {
-            'property1': stix2.properties.StringProperty(required=True),
-        })
+        @stix2.observables.CustomExtension(Foo, 'x-new-ext', [
+            ('property1', stix2.properties.StringProperty(required=True)),
+        ])
         class FooExtension():
             pass  # pragma: no cover
     assert str(excinfo.value) == "'observable' must be a valid Observable class!"
@@ -495,9 +495,9 @@ def test_custom_extension_invalid_observable():
     class Bar(stix2.observables._Observable):
         pass
     with pytest.raises(ValueError) as excinfo:
-        @stix2.observables.CustomExtension(Bar, 'x-new-ext', {
-            'property1': stix2.properties.StringProperty(required=True),
-        })
+        @stix2.observables.CustomExtension(Bar, 'x-new-ext', [
+            ('property1', stix2.properties.StringProperty(required=True)),
+        ])
         class BarExtension():
             pass
     assert "Unknown observable type" in str(excinfo.value)
@@ -506,9 +506,9 @@ def test_custom_extension_invalid_observable():
     class Baz(stix2.observables._Observable):
         _type = 'Baz'
     with pytest.raises(ValueError) as excinfo:
-        @stix2.observables.CustomExtension(Baz, 'x-new-ext', {
-            'property1': stix2.properties.StringProperty(required=True),
-        })
+        @stix2.observables.CustomExtension(Baz, 'x-new-ext', [
+            ('property1', stix2.properties.StringProperty(required=True)),
+        ])
         class BazExtension():
             pass
     assert "Unknown observable type" in str(excinfo.value)
@@ -520,21 +520,29 @@ def test_custom_extension_no_properties():
         @stix2.observables.CustomExtension(stix2.DomainName, 'x-new-ext2', None)
         class BarExtension():
             pass
-    assert "'properties' must be a dict!" in str(excinfo.value)
+    assert "Must supply a list, containing tuples." in str(excinfo.value)
 
 
 def test_custom_extension_empty_properties():
+    with pytest.raises(ValueError) as excinfo:
+        @stix2.observables.CustomExtension(stix2.DomainName, 'x-new-ext2', [])
+        class BarExtension():
+            pass
+    assert "Must supply a list, containing tuples." in str(excinfo.value)
+
+
+def test_custom_extension_dict_properties():
     with pytest.raises(ValueError) as excinfo:
         @stix2.observables.CustomExtension(stix2.DomainName, 'x-new-ext2', {})
         class BarExtension():
             pass
-    assert "'properties' must be a dict!" in str(excinfo.value)
+    assert "Must supply a list, containing tuples." in str(excinfo.value)
 
 
 def test_custom_extension_no_init_1():
-    @stix2.observables.CustomExtension(stix2.DomainName, 'x-new-extension', {
-        'property1': stix2.properties.StringProperty(required=True),
-    })
+    @stix2.observables.CustomExtension(stix2.DomainName, 'x-new-extension', [
+        ('property1', stix2.properties.StringProperty(required=True)),
+    ])
     class NewExt():
         pass
 
@@ -543,9 +551,9 @@ def test_custom_extension_no_init_1():
 
 
 def test_custom_extension_no_init_2():
-    @stix2.observables.CustomExtension(stix2.DomainName, 'x-new-ext2', {
-        'property1': stix2.properties.StringProperty(required=True),
-    })
+    @stix2.observables.CustomExtension(stix2.DomainName, 'x-new-ext2', [
+        ('property1', stix2.properties.StringProperty(required=True)),
+    ])
     class NewExt2(object):
         pass
 
@@ -594,3 +602,8 @@ def test_register_custom_object():
     stix2._register_type(CustomObject2)
     # Note that we will always check against newest OBJ_MAP.
     assert (CustomObject2._type, CustomObject2) in stix2.OBJ_MAP.items()
+
+
+def test_extension_property_location():
+    assert 'extensions' in stix2.v20.observables.OBJ_MAP_OBSERVABLE['x-new-observable']._properties
+    assert 'extensions' not in stix2.v20.observables.EXT_MAP['domain-name']['x-new-ext']._properties
diff --git a/stix2/v20/observables.py b/stix2/v20/observables.py
index 83600b0..39a8f19 100644
--- a/stix2/v20/observables.py
+++ b/stix2/v20/observables.py
@@ -979,6 +979,9 @@ def CustomObservable(type='x-custom-observable', properties=None):
                                      "is not a ListProperty containing ObjectReferenceProperty." % prop_name)
 
             _properties.update(properties)
+            _properties.update([
+                ('extensions', ExtensionsProperty(enclosing_type=_type)),
+            ])
 
             def __init__(self, **kwargs):
                 _Observable.__init__(self, **kwargs)
@@ -1030,12 +1033,10 @@ def CustomExtension(observable=None, type='x-custom-observable', properties=None
         class _Custom(cls, _Extension):
 
             _type = type
-            _properties = {
-                'extensions': ExtensionsProperty(enclosing_type=_type),
-            }
+            _properties = OrderedDict()
 
-            if not isinstance(properties, dict) or not properties:
-                raise ValueError("'properties' must be a dict!")
+            if not properties or not isinstance(properties, list):
+                raise ValueError("Must supply a list, containing tuples. For example, [('property1', IntegerProperty())]")
 
             _properties.update(properties)