A series of Fe-ZSM-5 catalysts prepared by improved liquid ion exchange (see part 1 [J. Catal. 231 (2005) 314]) containing 0.2-1.2 wt% Fe, with a systematically changing nature of Fe sites, was studied during the selective catalytic reduction (SCR) of NO with NH3 or isobutane or during the interaction with feed components by various ill Situ methods (EPR, UV-vis. and FTIR spectroscopy). The results were related to the catalytic behavior. Several types of isolated Fe3+ sites with different reducibility were identified. FTIR results revealed that reduction of NO proceeds via intermediate formation of adsorbed nitrato species, which are Subsequently reduced. Their formation requires the presence of Fe3+. Those Fe3+ ions that are permanently reduced to Fe2+ under reaction conditions probably do not contribute to catalytic activity. In general, the degree of steady-state Fe site reduction during NH3 SCR is markedly lower than that during isobutane SCR. which may be a reason for the lower activity of Fe-ZSM-5 in the latter reaction. UV-vis and EPR results suggest that isolated Fe3+ sites in octahedral coordination are more easily reduced than tetrahedral Fe3+. In contrast to some types of isolated Fe3+ species, FexOy clusters reoxidize much more rapidly than they reduce, and thus remain essentially trivalent under reaction conditions. Due to their higher oxidation potential, they cause undesired total oxidation of the reductant, particularly in the case of isobutane. (c) 2006 Elsevier Inc. All rights reserved.