CaO-CeO2 is the most effective catalyst for the conversion at 850 degrees C of CH4 to C2H6 and C2H4 by CO2 among a series of CeO2 catalysts modified with alkali and alkaline earth metal oxides. When the CaO-CeO2 catalyst is prepared in the range of Ca/Ce ratio 0.1-0.5 by impregnation method, there exist synergistic effects between the two components for the formation of C-2 hydrocarbons, and the catalyst forms solid solution. It is thus suggested that the formation of solid solution is responsible for synergistic effects. The lattice oxygen of the CaO-CeO2 catalyst converts CH4 mainly to H-2 and CO, and the presence of CO2 is indispensable for C-2 formation. Both C-2 selectivity and C-2 yield increase remarkably with increasing partial pressure of CO2, these values at 850 degrees C reaching 75 and 4% at 70 kPa, respectively. Correlation of the results of kinetic analyses and CO2 TPD measurements indicates that the existence of the pool of the CO2 chemisorbed on the CaO-CeO2 catalyst accounts for high C-2 selectivity and yield. The characterization of the binary catalyst show that bulk carbonates are not detectable during reaction and Ce3+ sites are formed at the outermost layer. It is speculated that these sites activate the chemisorbed CO2 to generate active oxygen species, which work for the conversion of CH4 to C-2 hydrocarbons.