Characterization of palladium catalysts, supported on alumina and alumina modified by zirconia added by impregnation, was performed by CO adsorption from 143 to 298 K and monitored by infrared spectroscopy. It was found that the population of the Al3+ octahedral sites in the alumina decreased by the addition of zirconia. In contrast to the case of the pure alumina support, where stabilization of Pd+ was observed, Pd2+ was formed preferentially on samples where zirconia was added, and higher crystallinity in the metallic palladium was observed. Studies of CO adsorption at low temperatures (143 K) gave a better description of the surface species, since at higher temperatures (298 K) the reaction of the CO with some of the palladium oxide particles led to the partial reduction of the latter.