Iron-containing zeolites with MFI structure are effective catalysts for several important reactions, among which the one-step hydroxylation of benzene to phenol by N2O stands out. We report on a multifrequency (9-475 GHz) continuous-wave electron paramagnetic resonance (CW-EPR) study of an Fe(III)-doped MFI zeolite catalyst either as synthesized and after detemplation, steaming, and treatment by aqueous dithionite. The presence of substantial inhomogeneity of the zero-field splitting (ZFS) interaction has been recognized and a novel computational strategy has been implemented to fit the spectroscopic results. This procedure enabled us to obtain quantitative measures of the ZFS distribution features and to draw structural conclusions regarding the nature and environment of Fe(III) ions in the as-synthesized catalyst and after each treatment.