The state of a series of palladium catalysts supported on ceria-zirconia and ceria-zirconia/alumina in the calcined state and under a reactive CO + O-2 atmosphere has been characterized by a combination of electron transmission microscopy, infrared and electron paramagnetic resonance spectroscopies, and the results obtained used to interpret the catalytic behavior displayed by the samples in the CO oxidation reaction. The Ce/Zr promoter enhances the catalytic properties of the noble metal by favoring the formation of the metallic state and through creation of anionic vacancies in the Ce-Zr component even from room temperature. The optimum promoting effect in the CO oxidation is observed for palladium particles in contact with three-dimensional-like Ce-Zr particles due to the appropriate nature and high number of the anion vacancies present at their surface. Compared with classical ceria-based systems, CO oxidation is here maximized for Pd-supported on the bulk material and not on mixed ceria-containing/alumina catalysts. These differences are discussed on the basis of the effect of the ceria and ceria-zirconia promoter particle size on the nature and number of active sites.