The reaction of [Mn(TF(4)TMAP)](CF3SO3)(5) (TF(4)TMAP-meso-tetrakis(2,3,5.6-tetrafluoro-N,N,N- trimethyl-4-aniliniumyl)porphinato dianion) with H2O2 (2 equiv) at pH 10.5 and 0 degreesC yielded an oxomanganese(v) porphyrin complex I in aqueous solution. whereas an oxomanganese(iv) porphyrin complex 2 was generated in the reactions of tert-alkyl hydroperoxides such as tert-butyl hydroperoxide and 2-methyl-1-phenyl-2-propyl hydroperoxide. Complex I was capable of' epoxidizing olefins and exchanging its oxygen with (H2O)-O-18. whereas 2 did not epoxidize olefins. From the reactions of [Mn(TF(4)TMAP)](5-) with various oxidants in the pH range 3-11, the O-O bond cleavage of hydroperoxides was found to be sensitive to the hydroperoxide substituent and the pH of the reaction solution. Whereas the O-O bond of hydroperoxides containing an electron-donating tert-alkyl group is cleaved homolytically, an electron-withdrawing substituent such as an acyl group in in-chloroperoxybenzoic acid (m-CPBA) facilitates O-O bond heterolysis. The mechanism of the O-O bond cleavage of H2O2 depends on the pH of the reaction solution: O-O bond homolysis prevails at low pH and O-O bond heterolysis becomes a predominant pathway at high pH. The effect of pH on O-18 incorporation from (H2O)-O-18 into oxygenated products was examined over a wide pH range, by carrying out the epoxidation of carbamazepine (CBZ) with [Mn(TF(4)TMAP)(5), and KHSO5 in buffered (H2O)-O-18 solutions. A high proportion of 110 was incorporated into the CBZ-10.11-oxide product at all pH values but this proportion was not affected significantly by the pH of the reaction solution.