The hydrothermal deactivation of Fe-ZSM-5 catalysts for the reduction of NO with NH3 in the presence of 10% H2O was studied. Aged and fresh catalysts were characterized by H-TPR, DRIFTS. Al-27 NMR, Si-29 NMR, XAS and nitrogen physisorption. The analysis of deactivated catalysts indicated that, under moderately aging conditions, dealumination due to hydrolysis occurs only on Al sites that bear a Bronsted acidic proton. Furthermore, the results showed that the cause of hydrothermal aging is Fe migration, which leads to the formation of FeOx clusters that increase in size as the age of the catalyst increases. This Fe migration was not strongly related to the process of dealumination and the stability with respect to SCR activity depends more on the stability of the active iron species in ion exchange sites than the stability of the framework itself. The hydrothermal stability of these active iron sites was not significantly influenced by the presence of Bronsted acid protons. The remaining activity after hydrothermal aging is caused by isolated iron ions located at ion exchange sites. Based on these results a general mechanism for aging under hydrothermal conditions was developed that differentiates between dealumination, migration and clustering of Fe oxides and the temporal sequence of these processes. 2010 Elsevier B.V. All rights reserved.