A catalyst composed of copper oxide supported on cerium-zirconium mixed oxide (CuOx/CeZrO4) has been studied with respect to its activity for CO oxidation under stoichiometric conditions employing either oxygen or oxygen with a small amount of NO as oxidant. The nature of copper oxide entities, as well as the redox properties of the catalyst following interactions with CO and O-2-NO, has been studied by XPS, EPR, and static NO adsorption infrared spectroscopy while in situ DRIFIFS has been employed to follow processes occurring at the catalyst surface under reaction conditions. Characterization of the copper oxide species in both fully oxidized and partially reduced states reveals that they are significantly affected by interactions with the underlying support. On the basis of catalytic activity results and in combination with analysis of the evolution of particular Cu+ carbonyls, comparing the CO oxidation reaction in the presence and absence of NO, it is proposed that the two basic factors affecting the catalytic performance of this type of system are the facility for achieving a partially reduced state for the copper oxide phase at the interfacial zone and the redox properties of the CuOx/CeZrO4 interface. (C) 2003 Elsevier Science (USA). All rights reserved.