A new (mu-oxo)diosmium(IV) complex with unsymmetric ligand environment, [Os-2(IV)(mu-O)Cl-5(CH3CO2)(py)(4)] (2), and its one electron reduced species (Et(4)N)[(OsOsIV)-Os-III(mu-O)Cl-5(CH3CO2>(py)(4)] ((Et(4)N)[3]) have been prepared and characterized by several physical methods. X-ray crystallography was conducted for 2 and (Et(4)N)[3] as well as the osmium(VI) complex [OsO2Cl2(py)(2)] (1). 1 takes a trans-dioxo structure with both the pyridine and chloro ligands in a cis configuration. The reaction of 1 with refluxing CH3COOH/(CH3CO)(2)O mixtures provided 2 with the linear Os-O-Os moiety in which trans positions to the oxide bridge are occupied by monodentate acetate for one osmium ion and chloride for another. Cyclic voltammetry showed that 2 is oxidized irreversibly at E(pa) = +1.68 V, and is reduced reversibly and quasi-reversibly at E(1/2)(1) = +0.03 and E(1/2)(2) = -1.15 V vs Ag/AgCl, respectively, in acetonitrile. The compound 2 is reduced chemically as well as electrochemically to the mixed-valence compound, [3](-). The complex anion [3](-) crystallizes with Et(4)N(+) in a 1:1 ratio. The average oxidation state of osmium ions is +3.5. The structure of [3](-) is very similar to that of 2 except that all bond lengths parallel to the Os-O-Os axis are elongated. Differences in the two Os-O distances are much more evident in 2 rather than [3](-). Equatorial Os-N and Os-Cl distances of the two osmium pseudooctahedra in 2 are respectively virtually identical and are only slightly changed on reduction to [3](-). No splitting of the peak was observed in the X-ray photoelectron spectrum of [3](-) in the Os(4f(7/2)) region. The odd electron in [3](-) is delocalized on the two osmium atoms. Crystal data : [OsO2Cl2(py)(2)], tetragonal, space group P4(1)2(1)2, a = 10.452(1) Angstrom, c = 12.149(1) Angstrom, Z = 4; [Os-2(IV)(mu-O)Cl-5(CH3CO2)(py)(4)], monoclinic, space group P2(1)/c, a = 18.007(2) Angstrom, b = 10.378(2) Angstrom, c = 15.529(2) Angstrom, beta = 104.53(1)degrees, Z= 4; (Et(4)N)[(OsOsIV)-Os-III(mu-O)Cl-5(CH3CO2(py)(4)], orthorhombic, space group Pcab, a = 20.895(4) Angstrom, b = 25.120(5) Angstrom, c = 14.149(9) Angstrom, Z = 8.