diff --git a/ontology/modules/observation/0.1/observation.ttl b/ontology/modules/observation/0.1/observation.ttl
index 3cd3ff6..e70531a 100644
--- a/ontology/modules/observation/0.1/observation.ttl
+++ b/ontology/modules/observation/0.1/observation.ttl
@@ -149,6 +149,7 @@ vann:preferredNamespaceUri rdf:type owl:AnnotationProperty .
 :observesDuring rdf:type owl:DatatypeProperty ;
                 rdfs:subPropertyOf <http://www.w3.org/2006/time#hasXSDDuration> ;
                 rdfs:domain :Observation ;
+                rdfs:range xsd:duration ;
                 rdfs:label "observes during" .