Cu-Ce-O binary oxide catalysts were found to have high activity for the oxidation of CO and methane by oxygen to carbon dioxide and water as well as oxidation of CO by sulfur dioxide to carbon dioxide and elemental sulfur. The catalysts were characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy. It was found that copper in the Cu-Ce-O system existed in the form of clusters dispersed in the cerium oxide matrix and in the form of segregated agglomerates (> 10 nm). The former are present in all catalyst compositions, while the latter increased with the copper content. The dispersed copper clusters may be stabilized by diffusion of copper ions into interstitial metal ion sites of the cerium oxide lattice forming Cu+1 species in the process. Cu+1 species were identified by XPS. A synergistic reaction model is proposed to explain the enhanced catalytic activity and stability. In the model, the Cu+1 species on the copper cluster provide surface sites for CO adsorption and the cerium oxide provides the oxygen source. The adsorbed CO reacts with oxygen at the interface of the two materials.