Fe-ZSM-5 are prepared by using four different techniques: conventional aqueous ion-exchange (CA), improved aqueous ion-exchange (IA), solid-state ion-exchange (SS) and chemical vapor ion-exchange (CV). All of the catalysts show very high activities for selective catalytic reduction (SCR) of NO with ammonia. However, the activities are different and follow the sequence of Fe-ZSM-5 (IA) > FeZSM-5 (CA), Fe-ZSM-5 (SS) > Fe-ZSM-5 (CV). ESR results indicate that Fe3+ ions with tetrahedral coordination are the active sites for the SCR reaction.