Catalytic activity tests in combination with characterization studies (using CO temperature programmed reduction, electron paramagnetic resonance, and Fourier transform infrared) have been performed with the aim of establishing which are the main factors influencing the reactivity of an oxidized Pt/CeO2/Al2O3 catalyst for the CO-O-2 reaction. Several effects are induced by the presence of both platinum and dispersed ceria in the catalyst. The results show that the low-temperature reducibility of both platinum and dispersed ceria is enhanced when the two components are present in the catalyst. Analysis of the characteristics of the centers responsible of the latter effect, carried out by means of EPR using oxygen as the probe molecule, indicates that in this sample the most reducible sites are formed by platinum located on bidimensional ceria patches present at the alumina surface. The enhancement by ceria of the low-temperature reduction of platinum at these sites under a CO + O-2 reacting mixture, observed even when starting from oxidized catalysts, leads us to propose these sites as highly active for the CO oxidation reaction.