Exposure to o(2) (or air) of a CH2Cl2, benzene, or toluene solution containing PhCO2H and Ru(OEP)(RR'S)(2) (where OEP = the dianion of 2,3,7,8,12,13,17,1 8-octaethylporphyrin, R = methyl, ethyl, or decyl, and R' = methyl or ethyl), at ambient conditions, results in the selective oxidation of the axial ligand(s) on the metalloporphyrin complex to the corresponding sulfoxide(s). For example, a CD2Cl2 solution of Ru(OEP)(dms)(2) (dms = dimethyl sulfide) and PhCO2H, exposed to 1 atm of O-2 at similar to 20 degrees C for 35 h, is oxidized to Ru(OEP)(dmso)(2), and the intermediates Ru(OEP)(dms)(dmso), [Ru(OEP)(dms)(2)][PhCO2], and Ru(OEP)(dms)(PhCO2) are identified (s implies sulfur-bonded). Mechanisms invoking in situ formation of H2O2, disproportionation of Rum species, and Ru-IV = O intermediates are proposed for the O-2 oxidation of the thioether ligands. X-ray analysis of Ru(OEP)(Et2SO)(2) confirms that the sulfoxides are S-bonded.