A study of the kinetics and mechanism of the selective catalytic reduction (SCR) of NO by NH3 over H-ZSM-5 was undertaken. Steady-state kinetic experiments performed at temperatures above 330 degrees C with and without the presence of H2O indicate that H2O does not affect SCR by NH3 at these temperatures. Analysis of the kinetic data indicates that SCR is positive order in NO and O-2 and inhibited by NH3. A series of transient tests were also performed to determine the roles of NO, O-2, and NH3 in the mechanism of NO SCR. Transient test results indicate that O-2 reacts with NO to form an active intermediate species (possibly NO2 or NO+) which reacts with adsorbed NH3. Transient test results also suggest that although adsorbed NH3 is necessary for NO reduction to proceed, excess gaseous NH3 inhibits SCR. Comparisons of SCR activity exhibited by samples of H-ZSM-5 and H-mordenite of different Si/Al ratios suggest that a limiting Al content is necessary for H-form zeolites to be active. This result suggests that pairs of neighboring Bronsted acid sites are necessary for adsorption of NH3, such that the adsorbed NH3 molecules are close enough together that they can both bond with other reactant molecules.