A step change technique was used to investigate individual reactions in the reduction of NO by hydrocarbon on Na-ZSM-5, H-ZSM-5, Cu/Na-ZSM-5 and Cu/H-ZSM-5 catalysts. Na-ZSM-5 is not active in NO decomposition and NO reduction by propane both in the presence and in the absence of oxygen. On the other hand, H-ZSM-5 is active in the oxidation of NO to NO2 and in the reduction of NO. Zeolite does not adsorb NO but preferentially adsorbs O-2 which, subsequently, reacts with NO in the gas phase to produce NO2. In the absence of oxygen, Cu/H-ZSM-5 is more active than Cu/Na-ZSM-5 in both the decomposition of NO and the reduction of NO by propane since Cu/H-ZSM-5 can produce more NO, in the absence of oxygen. Differences in the Cu/H and Cu/Na forms of ZSM-5 result from residual H+ or Na+ in these zeolites. These residual cations either affect catalytic sites directly or control the Cu+/Cu2+ ratios. Differences in these ratios were observed.