Many dinuclear dioxygen-bridged cobalt(III) complexes have been reported.(1) These compounds have been increasing in interest from the point of view of oxygen activation and selective or specific oxidation reactions for organic molecules.(2) Recently, we communicated the novel synthesis of the (mu-peroxo)(mu-thiosulfato)cobalt(III) complex [(tren)Co(mu-O-2)(mu-S2O3)Co-(tren)](2+) [tren = tris(2-aminoethyl)amine] from [CoCl2(tren)]Cl and NH2(CH2)(n)NH2.H2Sx in the presence of activated charcoal.(3) The X-ray crystal structure reveals that the complex has an unprecedentedly large Co-O-O-Co torsion angle of 75.5(2)degrees as compared with those of the known doubly bridged peroxo complexes [N4Co(mu-O-2)(mu-X)CoN4](n+) (X = OH- and NH2-).(1) It would be interesting to discover whether such a large torsion angle is essential for other mu-peroxo, mu-sulfur doubly bridged complexes. Here we report the improved synthesis of [(tren)Co(mu-O-2)-(mu-S2O3)Co(tren)](2+) and the related four peroxo complexes [(tren)Co(mu-O-2)(mu-SR)Co(tren)](n+) [SR = SCH2CH2SO32-,SCH2CH2OH-, SC6H5-, and SCH2CH2N(CH2CH3)(2)(-)]. These complexes were obtained in relatively good yields and showed characteristic UV/vis absorption spectra. X-ray crystal structures were determined for the two complexes with mu-S2O32- and mu-SC6-H-5(-). e.