[Mn-2(mu-O)(2)(mu-OAc)(H2O)(2)(bipy)(2)](ClO4)(3).H2O crystallizes in the monoclinic system, space group P2(1)/n (No. 14) with Z = 4 and a = 13.619(1) Angstrom, b = 16.213(2) Angstrom, c = 16.266(1) Angstrom, and beta = 113.08(1)degrees. The structure was solved by direct methods and refined to conventional agreement indices R = 0.021 and R(w) = 0.024 with 5159 unique reflections for which I > 3 sigma(I). The structure consists of binuclear [Mn-2(IV)(mu-O)(2)(mu-OAc)(H2O)(2)(bipy)(2)](3+) cations separated by ClO4- anions and a molecule of water of crystallization. This dimanganese(IV) compound is the first to include simultaneously a bridging acetato ligand, a bis(mu-oxo) bridge and two water molecules, each directly coordinated to one manganese(IV) ion. This high-valent bis(mu-oxo)-mu-acetato-bridged binuclear species has been studied with optical and EPR spectroscopy. The analysis of the thermal variation of its magnetic susceptibility affords one of the weakest antiferromagnetic interaction (J = -43.7 cm(-1), H = -2JS(1)S(2)) determined for a bis(mu-oxo)-bridged Mn-2(IV) species. The weakening of the antiferromagnetic interaction is tentatively correlated to the departure of the Mn-IV(mu-O)(2)Mn-IV ring from planarity (18.3 degrees) as a result of the constraint exerted by the mu-acetato bridge.