Catalysts with the same copper oxide loading supported on cerium oxide and Ce-M (M = Zr or Tb) mixed oxides are examined with respect to their performance for preferential oxidation of CO (CO-PROX) in a H-2-rich stream. The catalytic activity results are analysed in combination with operando-DRIFTS tests done under similar conditions and characterization results obtained by XRD, XPS and XAFS techniques. The presence of essentially highly dispersed CuO entities strongly interacting with the corresponding supports and differing in their dispersion degree is evidenced by these techniques. The catalytic activity results reveal significant support effects affecting the conversion of either CO or H-2 as well as the selectivity for the CO-PROX process. These are explained on the basis of differences in the interfacial redox activities of the corresponding copper oxide-support contacts, while copper oxide particle size effects are proposed to be relevant to explain overall CO-PROX selectivities obtained. (c) 2005 Elsevier B.V. All rights reserved.