Fe-ZSM5 was prepared with high iron content by solid-state ion exchange and characterized by ICP-AES, BET surface measurements, TEM, UV-vis. EPR and DRIFT spectroscopy as well as supplementing catalytic tests in order to clear up its functionality in urea-SCR. Due to the over-exchange with iron small Fe2O3 particles were formed, identified by UV-vis, EPR and TEM measurements, which were proved to be not active for the SCR reaction. However, the oxidation of NO to NO2 over Fe3+ ions in the catalyst was realized to be a pre-requisite for the SCR reaction and the rate-determining step. DRIFT investigations under SCR conditions showed adsorbates on Fe2+ up to 300 degrees C. The high SCR activity above 300 degrees C can be explained by the faster reoxidation of Fe2+ to Fe3+ sites at high temperatures. The observed inhibition of the SCR reaction by excess ammonia at low and intermediate temperatures can be explained in this context by the reducing properties of ammonia converting Fe3+ to Fe2+ or by preventing the reoxidation of Fe2+. (c) 2006 Elsevier B.V. All rights reserved.