Copper supported on ceria and ceria-magnesia catalysts were synthesized and used for NO reduction by CO. The supports and the subsequent Cu catalysts were prepared by citric acid sol-gel and impregnation methods, respectively. Based on XRD, TPR and EPR measurements, it is found that the Mg addition promotes not only the dispersion of CuO, but also the doping of copper ions into CeO2 matrix. A redox treatment favors some of copper ions in the octahedral sites of CeO2 substituting the lattice sites of Ce4+. Combining the results of activity test, we found that the formation of copper-substituted Cu-O-Ce solid solution promotes the catalytic activity of the catalysts. Also, the ways that MgO acts on the dispersion of CuO and the formation of Cu-O-Ce solid solutions were investigated with the aid of TEM, TGA and XRD techniques. The results of these studies suggest that magnesia crystallites undergo a phase evolvement during the preparation process of Cu/MgO-CeO2, leading to the reconstitution of the support and hence the improvement of copper dispersion, BET surface area, pore volume, as well as the doping of copper ions into the CeO2 matrix. (C) 2009 Elsevier B.V. All rights reserved.