This paper presents an approach for formalizing the RM-ODP (Reference Model for Open Distributed Processing), an ISO and ITU standard. The goal of this formalization is to clarify the RM-ODP modeling framework to make it more accessible to modelers such as system architects, designers and implementers, while opening the way for the formal verification of RM-ODP models, either within an ODP viewpoint or across multiple ODP viewpoints. Our formalization is based on set theory and the usual predicate logic, and is ex-pressed in the Alloy language.