Although atmospheric dioxygen is regarded as the most ideal oxidant, O-2 activation for use in oxygenation reactions intrinsically requires a costly sacrificial reductant. The present study investigated the use of aqueous alkaline solution for O-2 activation. A manganese(III) salen complex, Mn-III(salen)(Cl), in toluene reacts with aqueous KOH solution under aerobic conditions, which yields a di-mu-oxo dimanganese(IV) salen complex, [Mn-IV(salen)](2)(mu-O)(2). The O-18 isotope experiments show that O-18(2) is indeed activated to give [Mn-IV(salen)](2)(mu-O-18)(2) via a peroxide intermediate. Interestingly, the (OH-)-O-18 ion in H-2 O-18 was also incorporated to yield [Mn-IV(salen)](2) (mu-O-18)(2), which implies that a peroxide species is also generated from (OH-)-O-18. The addition of benzyl alcohol as a stoichiometric reductant selectively inhibits the O-18 incorporation from (OH-)-O-18, indicating that the reaction of Mn-III(salen)(Cl) with OH- supplies the electrons for O-2 reduction. The conversion of both O-2 and OH- to a peroxide species is exactly the reverse of a catalase-like reaction, which has a great potential as the most efficient O-2 activation. Mechanistic investigations revealed that the reaction of Mn-III(salen)(Cl) with OH- generates a transient species with strong reducing ability, which effects the reduction of O-2 by means of a manganese(II) intermediate.