Catalysts consisting of palladium supported on cerium dioxide (Pd/CeO2) were prepared and used for carbon monoxide oxidation in a stoichiometric mixture of carbon monoxide and oxygen. Pd/CeO2 exhibits high catalytic activity for the oxidation of CO, showing markedly enhanced catalytic activities due to the combined effect of palladium and cerium dioxide. The Pd/CeO2 catalyst is superior to Pd/ZrO2, Pd/Al2O3, Pd/TiO2, Pd/ZSM-5 and Pd/SiO2 catalysts with regard to the activity under the conditions examined. The catalysts were characterized by means of XRD and TPR. The position of the H-2-TPR peak shifts to lower temperature with increasing Pd loading from 0.25 to 2.0%. CeO2 inhibits the hydrogen reduction of PdO. CO-TPR measurements have shown the existence of three peaks. The low-temperature peak (alpha) is due to the Pd hydroxide species. The beta peak has been attributed to finely dispersed PdO. The high-temperature peak (gamma) has been attributed to crystal phase PdO. Crystal phase PdO is more difficult to reduce by CO than finely dispersed PdO. On the basis of the catalytic activity and CO-TPR results, we conclude alpha species (Pd hydroxide) mainly contribute to the catalytic activity for low-temperature CO oxidation.