A series of Fe-exchanged molecular sieves were studied as catalysts for the selective catalytic reduction (SCR) of NO with ammonia. It was found that Fe-ZSM-5 and Fe-mordenite catalysts were highly active for the SCR reaction. Nearly 100% NO conversions were obtained at 400-500 degrees C under conditions with a high space velocity (GHSV = 4.6 x 10(5) l/h). However, Fe-Y and Fe-MCM-41 with larger pore sizes showed lower activities for this reaction. For Fe-ZSM-5 catalysts, the SCR activity decreased with increasing Si/Al ratio in the zeolites. As the Fe-exchange level in the Fe-ZSM-5 catalysts was increased from 58 to 252%, NO conversion increased at lower temperatures (e.g., 300 degrees C), but decreased at high temperatures (e.g., 600 degrees C). Compared with the commercial vanadia catalyst, based on the first-order rate constants, the Fe-ZSM-5 catalyst was five times more active at 400 degrees C and seven times more active at 450 degrees C. It also functioned in a broader temperature window produced only N-2 (rather than N2O) and H2O, and showed a substantially lower activity for oxidation of SO2 to SO3.