Kinetics of CO oxidation on Pt is bistable both under UHV conditions and at atmospheric pressure. In the former case, the bistability can be explained by using the established reaction scheme, including reversible CO adsorption, dissociative O(2)adsorption, and reaction between adsorbed CO and O. Also at higher pressures, this model accounts well for the low-reactive (CO-dominated) regime. However, in the high-reactive (oxygen-rich) regime, the kinetics may be influenced by surface oxide formation at the higher pressures. Our present Monte Carlo simulations illustrate what may happen when oxide formation and removal occur with participation of gas-phase O-2 and CO molecules, respectively. Specifically, we show that these relatively slow steps may control the reaction rate in the high-reactive state. In this case, the hysteresis loop is found to be qualitatively similar to that predicted by the conventional reaction scheme, but the CO2 formation rate in this state is appreciably lower. These predictions are in qualitative agreement with the results of recent STM studies of CO oxidation on Pt(110) at atmospheric pressure. (C) 2003 Elsevier Inc. All rights reserved.