Partial oxidation of methane (POM) to synthesis gas, CO + H-2, was investigated between 50 degrees C and 700 degrees C as a function of metal loading over cerium and nickel based CeNixOy catalysts. At high temperature, 700 degrees C, a H-2 yield of 100% is obtained. At 200 degrees C, an interesting yield of 34% is reached with a stable methane conversion of 70% and a H-2 selectivity of 49%, only when the solid is previously in situ treated in H-2 at 200 degrees C. Activated in H-2 CeNixOy solids are hydrogen reservoirs. Different physico-chemical techniques were used to characterize the structural dispersion and reduction properties of the solids Depending on composition, a solid solution and/or a highly dispersed nickel oxide on ceria can be obtained. Ion sputtering followed by XPS analysis was very useful for estimating the size of small NiO crystallites present in the compounds. It is found that active nickel species belong to small clusters and/or to solid solution. Correlations among species present in solid and catalytic performances are discussed. An active site based on formation of anionic vacancies and a mechanism involving a heterolytic abstraction of a hydride species from methane are proposed. (c) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.