Redox processes occurring during the CO-O-2 reaction and catalytic properties for CO oxidation have been examined for CuO/CeO2 and CuO/ZrCeO4 catalysts using catalytic activity tests, in situ DRIFTS and EPR spectroscopies, and CO-TPD. EPR results suggest synergetic effects in the reduction-oxidation of these systems, both copper and the support being involved in these redox processes. Both EPR and CO-TPD experiments indicate a more facile reduction by CO of the CuO/CeO2 system, mainly due to the greater reducibility of sites at the interface between the copper oxide clusters and the support for this catalyst. A greater CO oxidation activity is exhibited by the CuO/CeO2 catalyst. A comparison of both catalysts and analysis of the redox properties of the different copper entities present in the catalysts indicate that the active copper sites for CO oxidation are located on the copper oxide clusters. On the basis of the results obtained by the different techniques, it is proposed that CO oxidation in these systems follows a redox mechanism in which both the support and copper oxide clusters are simultaneously reduced or oxidized following interaction with CO or O-2, respectively. The limiting step in the reaction is related to the oxidation process.