We have studied the reduction of NO2 by NH3 over HZSM-5 and find that this reaction is several hundred times faster than the corresponding reduction of NO under similar conditions. In addition, the kinetics of the reduction of NO2 are strikingly different from the kinetics of NO reduction. Whereas the reduction of NO is inhibited by the adsorption of ammonia and is first order in oxygen concentration, the rate of conversion of NO2 increases as NH3 concentration increases and is independent of oxygen concentration. We believe that these results strongly support our earlier suggestion that the oxidation of NO is the rate-determining step in the selective catalytic reduction of NO over HZSM-5.