The bistability of the kinetics of CO oxidation on Pt at sub-atmospheric pressures can be complicated by surface-oxide formation. We present a simple mean-field model making it possible to describe a transition from the low reactive reaction regime occurring via the conventional mechanism of CO oxidation at CO excess, to the high reactive regime including CO interaction with a fully developed surface-oxide overlayer at O-2 excess. In the latter case, the oxide is assumed to form islands, the CO2 formation may run primarily on oxide, and in agreement with recent experiments the reaction rate may be several orders of magnitude lower than the CO adsorption rate on a bare metal surface. (C) 2004 Elsevier B.V. All rights reserved.