We studied reactivity of well-defined Pt model catalysts, supported on crystalline iron oxide Fe3O4(1 1 1) films, in low temperature CO oxidation. It is shown that pre-annealing in vacuum at similar to 850 K suppresses CO adsorption but increases CO2 production rate. This finding is rationalised in terms of strong metal-support interaction between Pt and iron oxide resulting in particles encapsulation by a thin FeO(1 1 1) film that catalyses CO oxidation similarly to the extended FeO(1 1 1)/Pt(1 1 1) surfaces previously studied [10]. The results show that the stability and the atomic structure of the encapsulated layer under reaction conditions play a critical role in oxidation reactions over Pt catalysts supported on reducible oxides. (C) 2010 Elsevier B.V. All rights reserved.