Transition-metal peroxos have been implicated as key intermediates in a variety of critical biological processes involving O-2. Because of their highly reactive nature, very few metal-peroxos have been characterized. The dioxygen chemistry of manganese remains largely unexplored despite the proposed involvement of a Mn-peroxo, either as a precursor to, or derived from, O-2, in both photosynthetic H2O oxidation and DNA biosynthesis. These are arguably two of the most fundamental processes of life. Neither of these biological intermediates has been observed. Herein we describe the dioxygen chemistry of coordinatively unsaturated [Mn-II((SN4)-N-Me2(6-Me-DPEN))] (+) (1), and the characterization of intermediates formed en route to a binuclear mono-oxo-bridged Mn(III) product {[Mn-III((SN4)-N-Me2(6-Me-DPEN)](2)(mu-O)}(2+) (2), the oxo atom of which is derived from O-18(2). At low-temperatures, a dioxygen intermediate, [Mn((SN4)-N-Me2(6-Me-DPEN))(O-2)](+) (4), is observed (by stopped-flow) to rapidly and irreversibly form in this reaction (k(1)(-10 degrees C) = 3780 +/- 180 M-1 s(-1), Delta H-1(double dagger) = 26.4 +/- 1.7 kJ mol(-1), Delta S-1(double dagger) = -75.6 +/- 6.8 J mol(-1) K-1) and then convert more slowly (k(2)(-10 degrees C) = 417 +/- 3.2 M-1 s(-1), Delta H-2(double dagger) = 47.1 +/- 1.4 kJ mol(-1), Delta S-2(double dagger) = -15.0 +/- 5.7 J mol(-1) K-1) to a species 3 with isotopically sensitive stretches at v(O-O)(Delta O-18) = 819(47) cm(-1), k(O-O) = 3.02 mdyn/angstrom, and v(Mn-O)(Delta O-18) = 611(25) cm(-1) consistent with a peroxo. Intermediate 3 releases approximately 0.5 equiv of H2O2 per Mn ion upon protonation, and the rate of conversion of 4 to 3 is dependent on [Mn(II)] concentration, consistent with a binuclear Mn(O-2(2-)) Mn peroxo. This was verified by X-ray crystallography, where the peroxo of {[Mn-III((SN4)-N-Me2(6-Me-DPEN)](2)(trans-mu-1,2-O-2)}(2+) (3) is shown to be bridging between two Mn(III) ions in an end-on trans-mu-1,2-fashion. This represents the first characterized example of a binuclear Mn(III)-peroxo, and a rare case in which more than one intermediate is observed en route to a binuclear mu-oxo-bridged product derived from O-2. Vibrational and metrical parameters for binuclear Mn-peroxo 3 are compared with those of related binuclear Fe- and Cu-peroxo compounds.