In this study, the introduction of iron into the ZSM-5 zeolite from a water/acetonitrile solution of iron(III) acetylacetonate was studied, using the exchange process. The water and acetonitrile are both necessary to prepare effective Fe-ZSM-5 catalysts in the selective catalytic reaction of NO by NH3. The role of water is to favour the formation of low charged iron cationic species and the role of acetonitrile is to limit the hydrolysis reaction. Using a water/acetonitrile mixture of 75/25 vol:vol, 92 % of the iron in solution is retained by the zeolite at ambient temperature. The resulting catalyst contains iron species, reducible at lower temperatures, and exhibits a better efficiency in the reaction of nitrogen monoxide reduction by ammonia.