Copper supported on ceria prepared by the impregnation method was used as a model catalyst to investigate the synergetic mechanism between copper species and ceria for NO reduction by CO. To identify the copper species in the catalyst, we treated the as-synthesized Cu/CeO2 by 30 wt.% nitric acid solution. Fresh and nitric acid treated Cu/CeO2 catalysts were characterized and compared with AAS, XRD, TPR, EPR, Raman and NO-TPD techniques. It is found that, after nitric acid treatment, there is only 0.27 wt.% Cu left (vs. 5 wt.% Cu in fresh sample); this is present in the following forms: (1) isolated copper ions in octahedral sites of ceria, (2) copper oxide clusters and (3) copper ions in ceria lattice. The residual small amount of Cu has a significant effect on surface structure, redox property and catalytic behavior of ceria, indicating that these copper species are crucial copper species inducing copper-ceria interactions. Moreover, it is suggested that the relative free movement of oxygen from ceria to supported copper, caused by the three copper species, leads to oxygen vacancies in ceria, significantly enhancing the NO conversion of Cu/CeO2 during NO + CO reaction. (C) 2010 Elsevier B.V. All rights reserved.