The structure and reactivity of a series of Pt(111)/ceria model catalysts have been studied as a function of ceria coverage and morphology. XPS and LEED data show that the method of preparation results in growth of disordered CeO2 which converts to epitaxial (111)-oriented ceria upon annealing at >700 K. Submonolayer ceria coverages strongly promote the oxidation of CO. Most strikingly, the fully encapsulated Pt(111) wafer is a much more effective catalyst than the bare metal itself. Disordered encapsulating films are much more active than ordered films. The results are discussed in terms of Pt-promoted oxygen vacancy formation in ceria and the predicted relative reactivity of principal crystal faces of the oxide.