By replacing CH4 with CD4, the isotope effect on the reaction of adsorbed hydrocarbon species with CO2 over Ni/Al2O3 catalyst was studied using pulse surface reaction rate analysis (PSRA). The first-order rate constant for this step was 1.45 times larger for CH4 than for CD4. The observed isotope effect suggests that the reaction of adsorbed hydrocarbon species with CO2 (or adsorbed oxygen) is rate-controlling for the reforming of CH4.