The rate-determining step of the NO-CH4-O-2 reaction over Pd/H-ZSM-5 catalyst at 623-703 K was investigated mainly by the kinetic isotope effect, that is, the relative rates of the NO-CH4-O-2 and NO-CD4-O-2 reactions. Based on the isotope effect and its temperature dependency as well as the relative rates of NO-CH4-O-2 and NO2-CH4-O-2, it was concluded that the oxidation of NO to NO2 and the dissociation of a C-H bond of CH4 to form CHx are slow steps of comparable order and there is no dominant rate-determining step in the present reaction system. The relative rates of the two reactions vary depending on the reaction temperature; the oxidation of NO tends to be more rate-controlling at low temperatures. It is also probable under certain conditions that the reaction between NO2 and CHx subsequent to the above two reaction steps becomes a slow step.