Abstract: The oxidation of CO over Pt and Pt–Co catalysts was studied both experimentally and by quantum-chemistry calculations. Density functional theory simulation shows that CO is oxidized on Pt13 via dissociative oxygen adsorption. The calculated activation energy for the dissociation of O2 on Pt13 is 56 kJ/mol. It is found that when a cobalt atom is introduced into the cluster, the activation energy falls for the stages of CO oxidation and the dissociation of O2. Catalytic tests performed on Pt and Pt0.5Co0.5 nanopowders confirm the Pt–Co system displays high activity in the oxidation of CO.