The effect of ammonia supply on the selective catalytic reduction of NOX over zeolite H-ZSM-5 was investigated using step response experiments between 200 and 500 degreesC. For inlet NO:NO2 ratios > 1, the activity for NOX reduction transiently increased when NH3 was removed from the feed. For NO:NO2 ratios less than or equal to 1, the NOX reduction however decreased. By pulsing NH3 to the feed, the activity for NO reduction was enhanced up to five times compared to continuous supply of ammonia. For NO:NO2 ratios exceeding one, also the selectivity towards N2O formation was lower with transient ammonia supply. Temperature programmed reaction experiments with preadsorbed NH3 showed highest initial NOX reduction activity when ammonia had been adsorbed at 300 or 250degreesC compared to 200degreesC. A minimum in NO reduction was observed at 130 degreesC independent of the ammonia adsorption temperature. For NO:NO2 ratios > 1, the results strongly indicate that NO oxidation is the rate determining step in the ammonia selective catalytic reduction (NH3-SCR) reaction over H-ZSM-5. (C) 2003 Elsevier Inc. All rights reserved.