The effect of supports (ceO(2), TiO2, Al2O3, ZrO2, and SiO2) on CO oxidation over Pd catalysts at low temperatures was investigated, and activity-controlling factors were discussed. The light-off temperature was in the order of Pd/CeO2 < Pd/TiO2 < Pd/Al2O3 < Pd/ZrO2 < Pd/SiO2 which was fairly in agreement with the reduction temperature of supported Pd to metallic species in CO-Temperature programmed reduction (CO-TPR) profile. The result indicates that higher reducibility of supported Pd to metallic species is one of the activity-controlling factors for the oxidation activity. The activation of oxygen on metal oxide support is suggested to be another factor for the low temperature oxidation. Actually, in situ IR measurement revealed that the oxidation of CO proceeds on Pd/CeO2 and Pd/TiO2 even under nearly 100% coverage of CO over Pd, though Pd/Al2O3 showed CO oxidation activity below 50% coverage of CO. The supports having oxygen storage property, CeO2 and TiO2, contribute to the oxygen activation to proceed CO oxidation irrespective of self-poisoning of metallic Pd surface by adsorbed CO. It was concluded that both of the following two factors are essential for CO oxidation at low temperature: (1) formation of metallic Pd and (2) oxygen storage property of metal oxide support. (C) 2012 Elsevier B.V. All rights reserved.