The oxidation of CO by O2was studied for a Pt/¿-Al2O3catalyst and for a commercially available Pt/Rh/CeO2/¿-Al2O3three-way catalyst. Kinetic experiments were carried out in an isothermal fixed-bed microreactor under intrinsic conditions, i.e., in the absence of mass and heat transfer limitations, in the temperature range from 436 to 503 K, with CO and O2inlet partial pressures between 0.12 and 8.3 kPa and H2O and CO2inlet partial pressures between 0 and 10 kPa. For the Pt/¿-Al2O3catalyst, the CO2production rate was found to be essentially proportional to the oxygen and inversely proportional to the carbon monoxide partial pressures, although at large CO and small O2partial pressures deviations occur. A kinetic model, based on elementary reaction steps, was constructed. It was concluded that for the experimental conditions considered, the noble metal surface is almost completely covered with CO, the CO adsorption being in quasi-equilibrium, and that irreversible molecular adsorption of oxygen is the rate-determining step, followed by potentially instantaneous dissociation. The presence of steam was found to enhance the reaction rate. For the experiments carried out over Pt/Rh/CeO2/¿-Al2O3in the presence 10 kPa H2O and 10 kPa CO2, it was found that the CO2production rate becomes zero order in CO at high CO partial pressures. The partial reaction order in O2is approximately 0.5. The experimental observations were explained by the existence of a second bifunctional reaction path next to the reaction path catalyzed by the noble metal only. The bifunctional reaction path involves a reaction between CO adsorbed on the noble metal and oxygen from ceria at the noble metal/ceria interface. The experiments could be described adequately over the investigated range of conditions by a kinetic model incorporating the mono- and bifunctional reaction paths. For the quantification and understanding of the changes in the partial reaction orders in CO and O2as a function of the experimental conditions, a kinetic model based on elementary reaction steps is necessary.