Mechanistic studies of the aromatic hydroxylation by high-valent iron(IV)-oxo porphyrin π-cation radicals revealed thatthe aromatic oxidation involves an initial electrophilic attackon the π-system of the aromatic ring to produce a tetrahedralradical or cationic σ-complex. The mechanism was proposedon the basis of experimental results such as a large negativeHammett ρ value and an inverse kinetic isotope effect. Bycarrying out isotope labeling studies, the oxygen in oxygenated products was found to derive from the iron-oxoporphyrin intermediates.