By performing the CH4 + CO2 and CD4 + CO2 reactions alternately over SiO2-supported nickel catalysts in a pulse microreactor, normal deuterium isotope effects on both the methane conversion reaction and on the CO formation reaction have been observed in the process of CO2 reforming of methane. Based on the observed CH4/CD4 isotope effects, the pathways for the formation of CO are discussed.