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The effect of ammonia supply on the selective catalytic reduction of NO_x over zeolite H–ZSM-5 was investigated using step response experiments between 200 and 500°C. For inlet NO:NO₂ ratios > 1, the activity for NO_x reduction transiently increased when NH₃ was removed from the feed. For NO:NO₂ ratios ⩽ 1, the NO_x 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 NO_x reduction activity when ammonia had been adsorbed at 300 or 250°C compared to 200°C. A minimum in NO reduction was observed at 130°C independent of the ammonia adsorption temperature. For NO:NO₂ ratios …