CuO/Ce0.7Sn0.3O2, CuO/CeO2 and CuO/SnO2 catalysts were prepared using impregnation methods. The catalysts were characterized by means of H-2-TPR, X-ray diffraction (XRD) and CO-TPD studies. CuO/Ce0.7Sn0.3O2 catalysts have three reduction peaks, alpha, beta, and gamma. The alpha peak is attributed to the reduction of CuO and Sn4+ species on the surface of Ce0.7Sn0.3O2, beta peak to the reduction of bulk SnO2 and surface Ce4+, gamma peak to the reduction of bulk CeO2. Only a small amount of CuO (6%) is needed to form the active site for CO oxidation, and the excess CuO forms bulk CuO particles which contribute little to the activity. Combining the results of CO-TPD, XRD and catalytic activity measurements, we propose that the well-dispersed CuO, which can adsorb CO, is responsible for low-temperature CO oxidation. The bulk CuO that cannot adsorb CO contributes little to the oxidation activity. Among the supported CuO catalysts, a synergistic interaction between CuO and Ce0.7Sn0.3O2 makes the reduced CuO/Ce0.7Sn0.3O2 catalyst easily oxidized, thus it can easily supply active species. This is responsible for the highest CO oxidation activity at low-temperature. (C) 2003 Elsevier B.V. All rights reserved.