A series of xMO(y)/Ce0.9M0.1-xO2-delta (M = Cu, Ni, Co, Fe) catalysts (Ce-M) were prepared by a sol-gel method and corresponding Ce0.9M0.1-xO2-delta catalysts (HCe-M) were obtained with an acid treatment. It was found that the Ce-M catalysts contained surface MOy species and Ce0.9M0.1-xO2-delta solid solution, as confirmed by the results of X-ray diffraction and Raman spectra, respectively. While the HCe-M catalysts only contained Ce0.9M0.1-xO2-delta solid solution. CO oxidation over these catalysts revealed that the activities followed an order of Ce-Cu > Ce-Ni > Ce-Co > Ce-Fe, but were remarkably higher than the corresponding HCe-M samples. The activities of the HCe-M catalysts were closely related to their reducibility originated from the oxygen vacancies in these samples. The most active HCe-Cu catalyst had the highest content of oxygen vacancies and thus was the most reducible. For the Ce-M catalysts, the enhanced catalytic performance was due to a synergy between the surface MOy species and the Ce-M-O solid solution, as the former provided sites for CO chemisorption and the latter promoted the activation of oxygen. (C) 2013 Elsevier B.V. All rights reserved.