The reaction between CO2 and ZSM-5- and SiO2-supported Mo2C catalysts, found to be active in the aromatization of methane, has been studied in a flow and static system by measuring changes in the gas composition. The reaction was also followed with X-ray photoelectron spectroscopy. The decomposition of CO2 occurred above 823 K to give CO and O atoms. Using (CO2)-C-13 it was possible to establish that a fraction of O atoms oxidized the carbon of Mo2C into CO, the other, part reacted with Mo to give MoOx species. The ratio of (CO)-C-13/(CO)-C-12 varied between 5 and 6.0. XPS study confirmed the oxidation of Mo resulting in the formation of Mo ions of various oxidation states. Although the dominant oxidation state is Mo6+ at every temperature, Mo4+ and Mo5+ were present in the sample even after the reaction at 1073 K. The reaction proceeded at a faster rate on Mo2C/ZSM-5 than on Mo2C/SiO2.