Fe-ZSM-5 DeNO(x) catalysts prepared from H-ZSM-5 by different ion-exchange procedures have been analyzed by EPR and UV-VIS diffuse reflectance spectroscopy (DRS) ex situ after synthesis, calcination, and use in catalysis as well as in situ during calcination. The results have been correlated with the catalytic behavior of these materials in the selective catalytic reduction (SCR) of NO by isobutane or ammonia. In comparison to previous studies of the same samples by XAFS, XRD, XPS, TPR, and Mossbauer spectroscopy, the combination of EPR and UV/VIS-DRS was more sensitive for distinguishing between different types of isolated Fe species as well as FexOy aggregates of different size (oligonuclear clusters or large particles). It was found that aggregated species are formed at the expense of mononuclear Fe sites upon calcination at 873 K, and that aggregate formation is slightly favored by calcination with higher heating rates as well as by high Si/Al ratios of the parent H-ZSM-5. Use in SCR of NO leads to further growth and restructuring of FexOy clusters. From the comparison of structural and catalytic properties of different Fe-ZSM-5 catalysts it can be concluded that the SCR of NO by NH3 is catalyzed by different entities (mononuclear Fe sites, FexOy oligomers, surface of iron oxide particles). The results suggest that mononuclear Fe sites are also involved in the SCR with isobutane. Clustered sites, which may contribute to SCR with isobutane as well, appear to cause nonselective oxidation of the reductant (isobutane or ammonia) at higher temperatures. (C) 2004 Elsevier Inc. All rights reserved.