The relative reaction rates of CH4 and CD4 with NO in the presence of excess oxygen over a Co-ZSM5 catalyst have been determined. The rate ratio, CH4 consumption versus CD4 consumption, is nearly 2.4 when using a dry feed at 375°C. Experiments at 431°C using a feed containing 1.6% H2O show a rate ratio of 2.05. These values are close to those expected for a primary kinetic isotope effect at the respective temperatures, and it is therefore concluded that the rate-determining step is the breaking of a carbon hydrogen bond. The kinetic isotope effect is slightly less for the selective reaction with NO than for the concurrent direct oxidation of methane by oxygen. The two reactions have very similar kinetic orders in methane, suggesting that a common intermediate is involved in the two pathways. Hydrogen-deuterium exchange between CD4 and H2O to make CH3D is negligible under reaction conditions. Similarly, mixed feeds of CH4 and CD4 show very little exchange. One interpretation consistent with the results is that the rate-determining step involves detachment of hydrogen from methane by adsorbed nitrogen dioxide to form a methyl species which reacts further with a nitrogen oxide in one pathway but with oxygen in the other.