CO oxidation on a clean Pt(111) single crystal and thin iron oxide films grown on Pt(111) was studied at different CO:02 ratios (between 1:5 and 5:1) and partial pressures up to 60 mbar at 400-450 K. Structural characterization of the model catalysts was performed by scanning tunnelling microscopy, low energy electron diffraction, Auger electron spectroscopy and temperature-programmed desorption. It was found that monolayer FeO(111) films grown on Pt(111) were much more active than clean Pt(111) and nm-thick Fe3O4(111) films at all reaction conditions studied. Post-characterization of the catalysts revealed that at CO:O-2 > 1 the FeO(111) film dewets the Pt surface with time, ultimately resulting in highly dispersed iron oxide particles on Pt(111). The film dewetting was monitored in situ by polarization-modulated infrared reflection absorption spectroscopy. The reaction rate at 450 K exhibited first order for 02 and non-monotonously depended on CO pressure. In O-2-rich ambient the films were enriched with oxygen while maintaining the long-range ordering. Based on the structure-reactivity relationships observed for the FeO/Pt films, we propose that the reaction proceeds through the formation of a well-ordered, oxygen-rich FeOx (1 < x < 2) film that reacts with CO through the redox mechanism. The reaction-induced dewetting in fact deactivates the catalyst. The results may aid in our deeper understanding of reactivity of metal particles encapsulated by thin oxide films as a result of strong metal-support interaction. (C) 2009 Elsevier Inc. All rights reserved.