The selective oxidation of CO in a H-2-rich environment was studied over copper oxide supported on samaria-doped ceria (SDC). Activity tests, as well as H-2- and CO-temperature programmed reduction (CO-TPR), were carried out to seek insight into the effects of metal/support interaction occurring in the reaction. The interfacial active centers at the metal/support interfaces are inferred to be the major active sites of the reaction. By comparing the selective CO oxidation in Ar and in an atmosphere rich in H-2, we found that the conversion of CO was inhibited by H-2 at temperatures higher than 90 degreesC. In the H-2-rich atmosphere, the oxidation of CO and H-2 picked up considerably at 80 and 130 degreesC, respectively, due to differences in the reactivity of CO and hydrogen toward interfacial oxygen ions. Once bulk copper oxide is reduced by CO or H-2, the metastable states formed on the surface of the bulk CuO may serve as active sites, in addition to interfacial active centers, for the selective CO oxidation in excess H-2. CO has higher reactivity toward interfacial oxygen ions. Besides the interfacial active centers, the metastable copper clusters may also contribute to the occurrence of hysteresis in H-2 oxidation. Selectivity of the CuO/SDC catalyst results mainly from differences in the reactivity of CO and H-2 with these oxygen ions. By preparing SDC supports and CuO/SDC catalysts with different surface areas and dispersions, it was found that, although the better-dispersed catalysts exhibited higher CO conversions, more pronounced hydrogen oxidation was observed with these catalysts, thereby resulting in severer decline in selectivity.