Fe-exchanged ZSM-5 has been found previously to be much more active than commercial vanadia-based catalysts for selective catalytic reduction (SCR) of NO to N-2 with NH3. The NO reduction mechanism is studied in this work using combined in Situ FTIR (to observe surface-adsorbed species) and online mass spectrometry (to analyze reaction products). NH3 adsorbs quickly on Fe-ZSM-5 to generate NH4+ ions', and the catalyst is quite active in oxidizing NO to NO2 by O-2. The temperature-programmed surface reaction (TPSR) from 100 to 400degreesC shows that the reactivities of NH4+ with 4 NO and NO2 decrease in the order NO + NO2 (1: 1 ratio, producing N-2) > NO2 (producing N-2 + N2O) much greater than NO (producing N-2), with the same total NO, concentration for the three cases. This trend is also observed in their reactivities at 200degreesC. It is concluded that both NO and NO2 are involved in the reaction with NH4+ ions to 4 form N-2. Also, the reactivity of NH4+ ions with NO + O-2 on Fe-ZSM-5 is much higher than that on H-ZSM-5, but the two catalysts show the same activity for the reaction between NH4+ and NO + NO2. This suggests that NO, reduction probably takes place on the Bronsted acid sites and that the role of Fe3+ irons is to oxidize NO to NO,, by O-2. A possible reaction mechanism for NO reduction involves the reaction between one NO2 molecule and two neighboring NH4+ ions to form an active intermediate NO2(NH4+)(2), which subsequently reacts with another NO to produce N-2 and H2O. The intermediate has been detected by FTIR with a band near 1602 cm(-1).