diff --git a/src/main/cljc/org/domaindrivenarchitecture/activity_pub_poc/activitystreams2.cljc b/src/main/cljc/org/domaindrivenarchitecture/activity_pub_poc/activitystreams2.cljc
index 0a9a180..b90e766 100644
--- a/src/main/cljc/org/domaindrivenarchitecture/activity_pub_poc/activitystreams2.cljc
+++ b/src/main/cljc/org/domaindrivenarchitecture/activity_pub_poc/activitystreams2.cljc
@@ -14,7 +14,7 @@
 (s/def ::Object
   (s/and
    ::owl/Class
-   (sh/map-spec (s/keys :opt-un [::id ::type ::attachment ::bcc]))))
+   (sh/map-spec (s/keys :opt-un [::id ::type ::attachment ::bcc ::bto]))))
 
 ;http://www.w3.org/ns/activitystreams#Link
 ;TODO: definition in progress
@@ -67,6 +67,12 @@
          ::Object
          ::Link))
 
+;http://www.w3.org/ns/activitystreams#bto
+(s/def ::bto
+  (s/and ::owl/ObjectProperty
+         ::Object
+         ::Link))
+
 ;https://www.w3.org/TR/activitystreams-vocabulary/#dfn-type
 ;TODO: this can not be found in the ttl / namespace. Where is this exactly defined?
 (s/def ::type
diff --git a/src/test/cljc/org/domaindrivenarchitecture/activity_pub_poc/activitystreams2_test.cljc b/src/test/cljc/org/domaindrivenarchitecture/activity_pub_poc/activitystreams2_test.cljc
index faa5266..22c7596 100644
--- a/src/test/cljc/org/domaindrivenarchitecture/activity_pub_poc/activitystreams2_test.cljc
+++ b/src/test/cljc/org/domaindrivenarchitecture/activity_pub_poc/activitystreams2_test.cljc
@@ -48,6 +48,8 @@
 (deftest bcc-test
   (is (s/valid? ::sut/bcc ["http://joe.example.org"])))
 
+(deftest bto-test
+  (is (s/valid? ::sut/bto ["http://joe.example.org"])))
 
 (deftest Activity-test
   (is (s/valid? ::sut/Activity