The paramagnetic dinuclear Ru2+Ru3+ complex, {RuCl[P(OMe)(3)](2)}(2)(mu-S-2)(mu-Cl)(mu-N2H4) (1), reduces dioxygen by one-electron at room temperature in CH3CN to form 1(+)O(2)(-), in which O-2(-) is; coordinated to the mu-S-2 ligand. The dioxygen reduction is preceded by protonation to yield HO2+ by the bridging hydrazine ligand. Complex 1 is electrochemically more negative than HO2+, and a redox reaction generates 1(+) and HO2 (--> O-2(-) + H+). Spectroscopic studies (NMR and UV-vis) show that 1 and 1(+)O(2)(-) are under equilibrium, and the succeeding disproportionation reaction of 1(+)O(2)(-) produces two intermediates, 1(+) and the dithionite complex, 1(+)(O-2(-))(2), as an intermediate. The dithionite intermediate undergoes Cl- dissociation and further oxidation to yield the dinuclear Ru complex {Ru2+Cl[P(OMe)(3)](2)}(2)(mu-S2O5)(mu-N2H4) (3) The Liberated Cl- attacks the dithionite complex, and induces SO42- dissociation to prepare the dinuclear complex {Ru3+Cl2[P(OMe)(3)](2)}(2-) (mu-S)(mu-N2H4) (5). On the other hand, intermediate 1(+) forms an ion pair with HSO4-, which stems from the liberated SO42-, to yield [{Ru3+Cl[P(OMe)(3)](2)}(2)(mu-S-2)(mu-Cl)(mu-N2H4)]+HSO4- (4 = 1(+)HSO(4)(-)). A trace of {Ru3+Cl[P(OMe)(3)](2)}(2)(mu-S-2)(mu-Cl)(2) (2) is also detected in the supernatant. The UV-vis spectroscopic study revealed that complexes 2-5 are the exclusive products and the oxidation reaction can be balanced as follows: 14 x 1 + 4.5 O-2 = 2 + 5 x 3 + 4 x 4 + 4 x 5, The O-18(2)/O-16(2) mixed-labeled oxygen studies proved that the external oxygen was the source of oxygen for the formation of the S2O5 and HSO4- groups in 3 and 4, respectively. The structures of 3, 4, and 5 are characterized by X-ray crystallographic structure determinations. The dinuclear framework of complex 4 remains identical with that of 1 except for the significantly shorter Ru-S distance. The hydrazine hydrogens of complex 3 show intramolecular hydrogen bonding, which is reflected on a significantly low field shift of the hydrazine H-1 NMR resonance. Complex 3 possesses the mu-S2O52-ligand, in which the two mu-SO2-O,S' bridges are linked at the sulfur atoms through the mu-oxo bonds. Comparison of the site symmetries around the Ru atoms of 1, 3, and 5 strongly supports the occurrence of coordination rearrangements.