Although the Unified Modeling Language (UML) is by now accepted as a standard and is widely used as an object-oriented modeling language, some languages such as Z or VDM are preferred to formally specify reference models. Here, we present a reference model for hypermedia systems in UML. It is an object-oriented and Dexter-based approach that uses UML to provide an intuitive graphical representation of the model. The class diagrams are supplemented with formal constraints in the Object Constraint Language (OCL), adding invariants on elements as well as preconditions and postconditions on operations. The result is an easily extendable reference model for describing specific hypermedia systems, for example, like adaptive or mobile systems. Our contribution shows how formal and semiformal object-oriented techniques can be integrated in the metamodeling process.