A new diMn(III) complex, Na[Mn-2(3-Me-5-SO3-salpentO)(mu-MeO)(mu-ACO)(H2O)]center dot 4H(2)O (1), where salpentOH = 1,5-bis(salicylidenamino) pentan-3-ol, was synthesized and structurally characterized. The complex possesses a bis(mu-alkoxo)(mu-acetato) triply bridged diMn(III) core, the structure of which is retained upon dissolution. Complex 1 is highly efficient to disproportionate H2O2 in an aqueous solution of pH >= 8.5 or in DMF, with only a slight decrease of activity. Electrospray ionization mass spectrometry, EPR, and UV-vis spectroscopy used to monitor the H2O2 disproportionation in buffered basic medium, suggest that the major active form of the catalyst during cycling occurs in the Mn-2(III) oxidation state and that the starting complex retains the dinuclearity and composition during catalysis, with the acetate that moves from bridging to terminal ligand. UV-vis and Raman spectroscopy of H2O2 + 1 + Bu4NOH mixtures in DMF suggest that the catalytic cycle involves Mn-2(III)/Mn-2(IV) oxidation levels. At pH 10.6 in an Et3N/Et3NH+ buffer, complex 1 catalyzes dismutation of H2O2 with saturation kinetics on the substrate, first order dependence on the catalyst, and k(cat)/K-M = 16(1) x 10(2) s(-1) M-1. During catalysis; the exogenous base contributes to retain the integrity of the bis(mu-alkoxo) doubly bridged diMn core and favors the formation of the catalyst-peroxide adduct (low value of K-M), rendering 1 a highly efficient catalyst for H2O2 disproportionation.