A series of transient kinetic, semi-steady-state kinetic and in situ Fourier transform infrared spectroscopy (FTIR) experiments were conducted using NH3-presorbed H-form pentasil zeolites to reduce NOx to N-2. Semi-steady-state kinetic experiments were performed at temperatures below 210 degrees C to determine reaction rate parameters for the reduction of NO over NH3-presorbed H-mordenite. Transient test results suggest that pairs of NH3 molecules adsorbed onto neighboring acid sites are necessary for NO reduction to N-2. Results of in situ FTIR experiments were used to identify the structures of adsorbed NH3 and intermediate NOx species. These FTIR results suggest that adsorbed NH3 is most reactive when it is bonded to the Bronsted acid sites in zeolites through three hydrogen bonds. lit situ FTIR spectra also indicate that an NO2-type intermediate is formed on the zeolite surface during selective catalytic reduction (SCR). Formation of this NO2-type species appears to be a necessary step in the SCR reaction mechanism as the concentration of adsorbed NH3 does not decrease (i.e., react) until a band corresponding to this NO2-type species appears. Combined analyses of transient and in situ FTIR results were used to develop a reaction mechanism which describes NO or NO2 reduction by NH3 over H-form zeolites.