ABSTRACT A series of Ce20Cu5NiyOx catalysts for CO oxidation at low temperature were prepared and characterized by N2 adsorption, X-ray diffraction, temperature-program reduction by H2, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Ce20Cu5Ni0.4Ox exhibited the highest catalytic activity. The addition of NiO increased the amount of copper ions doped into the CeO2 matrix and gave more oxygen vacancies in ceria by the formation of a Ni-O-Ce solid solution. XPS results showed that large quantities of Cu+, Ce3+, and lattice oxygen existed in the fresh Ce20Cu5Ni0.4Ox catalyst. Cu+ ions in the catalyst can easily migrate to the ceria lattice to form a Cu-O-Ce solid solution, which enhanced the release of the lattice oxygen of the oxides under a reducing atmosphere. The high catalytic activity of Ce20Cu5Ni0.4Ox is due to the promoter giving increased amounts of Cu+ in the catalyst and the formation of solid solutions of both Cu-O-Ce and Ni-O-Ce.