In situ ESR of FeHZSM-5 shows that samples made by sublimation of FeCl3 contain mainly ferric ions in tetrahedral and distorted tetrahedral sites. Above 200 degrees C these ions do not chemisorb water, which confirms the resistance to activity deterioration by water vapor claimed for FeHZSM-5, when used in SCR of NOx. The number of ESR- active Fe3+ ions decreases when moving from strongly oxidizing conditions towards stoichiometry at 500 degrees C. In more reducing atmospheres, i.e., when excess reductant is present, the exposure produces an agglomerated ferromagnetic species, presumably magnetite. This process is irreversible. As the trivalent dispersed iron ions are the catalytically active sites, this transformation makes Fe-containing catalysts vulnerable to accidental but irreversible transformation induced by even mildly reducing conditions. The dispersion of ferric ions in FeSAPO-34 is not as good as in FeHZSM-5 and the reactive Fe3+ in distorted tetrahedral sites is absent. Catalytic oxidation of ethane allows correlation of the EPR results with activity and stability.