Fe-exchanged ZSM-5 (Fe/Al = 0.193), the most active catalyst known for the selective catalytic reduction (SCR) of NO with ammonia, was characterized by X-ray photoelectron spectroscopy (XPS), H-2 temperature-programmed reduction (H-2-TPR), electron spin resonance (ESR), and FT-IR spectroscopy. XPS and ESR results indicated that iron cations were present mainly as Fe3+ ions with tetrahedral coordination, along with a small amount of Fe2+ and aggregated Fe3+ ions. The Fe3+ ions could be partially reduced to Fe2+ ions by H-2 at 300-600 degrees C, but the oxidation was reversible when O-2 was introduced into the reduced catalyst at 500 degrees C. FTIR spectra showed that NO molecules could be oxidized by O-2 to adsorbed N2O3, NO2, and nitrate. The NOx adspecies were not stable at above 300 degrees C in He, but the adsorbed NO and NO2 could be observed in flowing NO + O-2/He. NH3 molecules were adsorbed on Bronsted acid and Lewis acid sites of Fe-ZSM-5 to generate, respectively, NH4+ ions (majority) and coordinated NH3 (minority). The NH4+ ions with three hydrogen atoms (3H structure) bonded to AlO4 tetrahedra of ZSM-5 were more stable at high temperatures (e.g., 300-400 degrees C) than those with two bonds and the coordinated NH3. The pretreatment by SO2 + O-2 at 400 degrees C increased the Bronsted acidity of the Fe-ZSM-5 due to formation of surface sulfate species of iron. This resulted in an enhancement of SCR activities by the presence of H2O + SO2 at high temperatures (>350 degrees C). At 300 degrees C, the NH4+ ions with 3H structure were active in reacting with NO and NO + O-2, but the reaction rate with NO + O-2 was much higher than that with NO. The results indicate that NH4+ ions with 3H structure as well as NO and NO2 adspecies play an important role in the SCR reaction on the Fe-ZSM-5 catalyst. The role of Fe3+ is to oxidize NO to NO2. Overexchange decreased the activity; the optimal Fe/Al ratio was 0.19-0.43.