We have studied the kinetics of the selective catalytic reduction (SCR) of NO by NH3 over HZSM-5. We find that between 350 and 450 degrees C the reaction is first order in NO and oxygen concentration but is negative order in ammonia. The rate data can be modeled by assuming that ammonia inhibits the reaction by blocking sites necessary for NO oxidation, which appears to be the rate-determining step. Ammonia oxidation is measurable at all conditions studied and becomes increasingly important as temperature and oxygen concentration increase or as NO concentration decreases; the oxidation of ammonia is more likely to produce N-2 than NO. We have developed a kinetic model that accurately describes the rate of N-2 formation over a wide range of conditions. We suggest that the active site for NO oxidation may be highly acidic extra-framework aluminum rather than the framework Bronsted aluminum sites. (C) 2000 Academic Press.