A case of apparent deactivation of a Pt SiO2 catalyst for the oxidation of a carbon monoxide during long periods on stream at 450 K has been studied using a combined infrared cell/flow reactor. When the catalyst was precovered by carbon monoxide, stable rates and high carbon monoxide coverages were observed from the commencement of reaction. However, direct exposure of reduced catalyst to carbon monoxide/oxygen mixtures resulted in much higher initial rates and a significantly lower initial carbon monoxide coverage. The activity subsequently declined and the coverage increased reaching the steady values characteristic of catalysts precovered by carbon monoxide after 24 h. It is believed that direct exposure to mixtures creates a ratio of oxygen coverage to carbon monoxide coverage which is much higher than the equilibrium one. The adsorbed CO forms patches and the oxygen adatoms exhibit a wide spread of activity. The catalytic rate is slow within CO patches and faster at the edges where oxygen adatoms are located. The apparent deactivation is caused by loss of the latter sites as the oxygen adatoms initially laid down are gradually consumed and replaced by adsorbed CO.