The oxidation and reduction of Pd/ceria and Pd/zirconia catalysts were investigated using pulse-reactor studies in which products were monitored following step changes in the reactant composition. The results demonstrate that both ceria and zirconia can be partially reduced by CO and partially reoxidized by CO2. Steady-state rates for methane reforming by CO2 were also measured on Pd/ceria, Pd/zirconia, and Pd/silica. The Pd/ceria catalyst was slightly more active than Pd/zirconia and similar to 10(4) times as active as Pd/silica. The high activities of Pd/ceria and Pd/zirconia are explained as resulting from oxidation and reduction of the supports. (C) 2000 Academic Press.