The combined partial oxidation and CO2 reforming of methane to synthesis gas was investigated over the reduced Co/MgO, Co/CaO, and Co/SiO2 catalysts. Only Co/MgO has proved to be a highly efficient and stable catalyst. It provided about 94-95% yields to H-2 and CO at the high space velocity of 105 000 ml g(-1) h(-1) and for feed ratios CH4/CO2/O-2 = 4/2/1, without any deactivation for a period of study of 110 h. In contrast, the reduced Co/CaO and Co/SiO2 provided no activity for the formation of H-2 and CO. The structure and reducibility of the calcined catalysts were examined using X-ray diffraction and temperature-programmed reduction, respectively. A solid solution of CoO and MgO, which was difficult to reduce, was identified in the 800 degreesC calcined MgO-supported catalyst. The strong interactions induced by the formation of the solid solution are responsible for its superior activity in the combined reaction. The effects of reaction temperature, space velocity, and O-2/CO2 ratio in the feed gases (while keeping the C/O ratio constant at 1/1) were investigated over the Co/MgO catalyst. The H-2/CO ratio in the product of the combined reaction increased with increasing O-2/CO2 ratio in the feed.