Heats of adsorption were measured calorimetrically for O-2 adsorption on reduced Pd/alumina, Pd/ceria, and Pd/ceria-zirconia catalysts, all with 1 wt% Pd. Significantly more O-2 adsorbed on the ceria-containing catalysts due to oxidation of the support. For Pd/alumina, the heats were found to be between 180 and 220 kJ/mol, only slightly higher in magnitude than the heat of reaction for bulk oxidation of Pd. However, the heats of adsorption for both ceria and ceria-zirconia were also similar to 200 kJ/mol, much lower than the heat of reaction for Ce2O3 oxidation to CeO2, but in reasonable agreement with estimates from O-2 desorption studies on model ceria films. The implications of these results for understanding oxygen-storage properties on ceria-based catalysts are discussed.