A method for the quantitative determination of isolated Fe3+ ions in FeHZSM-5 by EPR was devised. The method is based on the comparison of signals, taken at -196 degrees C, from pyridine-impregnated catalysts with signals from solutions of Fe(III)-phthalocyanine in pyridine. FeCl3 was introduced into HZSM-5 having Si/Al ratios of 15 and 25. The amount of Fe was between 0.5 and 5.5 wt%. The samples were prepared either by sublimation of FeCl3 into the zeolite in an N-2 flow or by impregnation with FeCl3 water solutions, and subsequently calcined at 400-800 degrees C. The measurement of "ESR-visible Fe3+" in FeCl3/HZSM-5 calcined at 400 degrees C in vacuum demonstrates that virtually all ferric ions, present as isolated FeCl2+-species, contribute to the ESR signal in samples with Fe/Al ratios <0.6. Introduction of iron in HZSM-5 by FeCl3 sublimation in an N-2 flow results in the stabilization of the same amount of isolated FeCl2+ species with the excess forming aggregates. The number of isolated Fe3+ ions decreases sharply after treatment in an air flow at T greater than or equal to 500 degrees C. Irrespective of the starting concentration of iron, after oxidative calcinations at 500-520 degrees C, the amount of isolated low-coordinated Fe3+ cations in FeHZSM-5 does not exceed Fe/Al similar to 0.15; the rest aggregates into ESR-invisible species or forms bulk alpha-Fe2O3. Calcination of FeHZSM-5 with the lower Si/Al ratio of 15 at 800 degrees C causes further loss of ESR-visible isolated Fe3+ ions, whereas the higher silica samples, with Si/Al = 25, are tolerant to this treatment. Formation of aggregated iron species is minor in the most diluted, low-loaded sample, 0.5% Fe/HZSM-5(Si/Al = 25), where the majority of Fe3+ (similar to 2/3) remains well isolated and contributes to the ESR signal even after oxidative treatment at 800 degrees C. Redox treatment of this sample even under mild conditions significantly redistributes the iron, with irreversible loss of of isolated Fe3+ ions and formation of ferromagnetic aggregates Of Fe3O4.