RDDOnto is an ontology that translates the MPEG-21 RDD (Rights Data Dictionary) specification into a hierarchical set of definitions with semantic content included. In the event that this set of definitions is used, the RDDOnto must provide well-defined semantics to determine which rights apply to data at all points within the hierarchy. RDDOnto translates the RDD specification into a machine-readable semantic engine that enables automatic handling of rights expressions. The Terms defined in the RDD Specification are what is going to be modelled using OWL (Web Ontology Language). For each Term, its description is composed by a set of descriptive attributes. With OWL, all the RDD relations between a term and other terms that capture its semantics have been mapped to RDDOnto. The specification of MPEG-21 RDD using OWL has also allowed to verify the consistency of the dictionary. 1