Three Fe-ZSM-5 catalysts (0.15-0.46 wt.% Fe) prepared via different routes with Fe ions present in extra-framework single sites as well as in oxidic clusters have been catalytically tested and monitored by operando EPR and UV-vis spectroscopy under standard and fast SCR conditions. In both cases, Fe ions in single alpha positions and oxidic clusters remain essentially trivalent while significant differences are evident for the remaining sites. During standard SCR, Fe sites in beta positions are completely, and those in gamma positions are partly reduced to inactive Fe-II, which is not able to catalyze the oxidative activation of NO being essential for its subsequent reduction to N-2. During fast SCR, the same beta and gamma sites are effectively reoxidized by NO2 and are thus kept in a redox-active state. The high reaction rates in fast SCR already at low temperatures are therefore assigned to those sites. (C) 2014 Elsevier Inc. All rights reserved.