Abstract In situ X-ray diffraction (XRD), in situ X-ray photoelectron spectroscopy (XPS), electron microscopy and thermogravimetric techniques were used to characterize CeM x O y mixed oxides in the oxidized and H 2 reduced state. The processes occurring during the hydrogen treatment, and the amounts of hydrogen occluded in the solids when treated at different temperatures under flowing hydrogen have also been determined. The CeM x O y oxides can be described as a mixture of copper oxide or nickel oxide and of ceria modified by the insertion of a part of copper or nickel in its lattice. The size of the copper oxide or nickel oxide varies considerably from clusters to a crystallized material, depending on the x value and on the experimental conditions. In the partially reduced state anionic vacancies able to receive hydrogen, probably in a hydridic form, are created in the bulk and at the surface of the solid and a reduction mechanism partly based on heterolytic dissociation of H 2 is proposed (O 2−+□+H 2→H −+OH −). Moreover, it appears that different kinds of active sites which differ from each other in terms of the environment of nickel or copper can exist.