The oxygen atom transferâ electron transfer (ET) mechanistic dichotomy has been investigated in the oxidation of a number of aryl sulfides by H2O2 in acidic (pH 3) aqueous medium catalysed by the water soluble iron(iii) porphyrin 5,10,15,20-tetraphenyl-21H,23H-porphine-p,pâ ²,pâ ³,pâ ´-tetrasulfonic acid iron(iii) chloride (FeTPPSCl). Under these reaction conditions, the ironâ oxo complex porphyrin radical cation, P⁺.rad; Fe(iv).dbd;O, should be the active oxidant. When the oxidation of a series of para-X substituted phenyl alkyl sulfides (X = OCH3, CH3, H, Br, CN) was studied the corresponding sulfoxides were the only observed product and the reaction yields as well as the reactivity were little influenced by the nature of X as well as by the bulkiness of the alkyl group. Labelling experiments using H2¹⁸O or H2¹⁸O2 clearly indicated that the oxygen atom in the sulfoxides comes exclusively from the oxidant. Moreover, no fragmentation products were observed in the oxidation of a benzyl phenyl sulfide whose radical cation is expected to undergo cleavage of the β Câ H and Câ S bonds. These results would seem to suggest a direct oxygen atom transfer from the ironâ oxo complex to the sulfide. However, competitive experiments between thioanisole (E° = 1.49 V vs. NHE in H2O) and N,N-dimethylaniline (E° = 0.97 V vs. NHE in H2O) resulted in exclusive N-demethylation, whereas the oxidation of N-methylphenothiazine (10, E° = 0.95 V vs. NHE in CH3CN) and N,N-dimethyl-4-methylthioaniline (11, E° = 0.65 V vs. NHE in H2O) produced the corresponding sulfoxide with complete oxygen incorporation from the oxidant. Since an ET mechanism must certainly hold in the reactions of 10 and 11, the oxygen incorporation experiments indicate that the intermediate radical cation, once formed, has to react with PFe(iv).dbd;O (the reduced form of the ironâ oxo complex which is formed by the ET step) in a fast oxygen rebound. Thus, an ET step followed by a fast oxygen rebound is also suggested for the other sulfides investigated in this work.