The interaction among the reacting species in the NO-CO-O-2 reaction on a metal catalytic surface that proceeds according to the Langmuir-Hinshelwood mechanism is studied by means of Monte Carlo simulation. The study of this three-component system is essential for the understanding of the influence of NO/O-2 ratio on the catalytic reduction of NO into N& O and oxidation of CO to CO2. It is found that this complex system, which has not been studied on these lines before, exhibits irreversible phase transitions between active states with sustained reaction and poisoned states with the catalytic surface fully covered by the reactants. The phase diagrams of the surface coverage with CO, N or O and the steady state production of CO2 are evaluated as a function of the partial pressure of CO in the gas phase. From this study, it is observed that with the addition of NO in the CO-O-2 reaction, the critical points in the phase diagram move towards lower values of CO partial pressure but the width of reaction window remains almost the same. However, the maximum production rate of CO2 decreases continuously. On the other hand, the addition of O-2 in the NO-CO reaction shifts the critical points towards higher values of CO pressure. Moreover, the width of reaction window as well as the production rate of CO2 increases with the increase in O-2 concentration. (c) 2007 Elsevier B. V. All rights reserved.