We investigate the catalytic reaction of CO molecules over a precovered surface of O atoms. The topmost surface is subject to a continuous flux of CO molecules and the oxidation reaction occurs through the Langmuir-Hinshelwood mechanism. The reactions only occur when a CO molecule finds an active surface O atom. The subsurface monolayers act as a reservoir of oxygen atoms, which guarantees the continuous evolution of the surface reactions. We have employed Monte Carlo simulations to monitor the CO2 yielding as a function of time. The transient profile of CO2 yielding depends on temperature, as well as on the initial distribution of O atoms on the subsurface planes. When the CO2 yielding attains a stationary state, an almost free oxygen region appears between the topmost layer and the bulk, whatever the initial distribution of oxygens we have considered. Our results are compared with some experimental observations at the oxygen-rich Ruthenium surface. (C) 2003 American Institute of Physics.