Resonance Raman (RR) and optical spectroelectrochemical titrations of the cis,cis-[(bpy)(2)Ru(OH2)](2)O4+ ion (denoted [3,3] to indicate the formal oxidation state of the Ru-O-Ru unit) were made over the range 0.8-2.0 V vs Ag/AgCl in 0.5 M trifluoromethanesulfonic acid; the results revealed sequential accumulation of three higher oxidation stares. Two of these states were identified by redox titration with Os(bpy)(3)(2+) as one-electron ([3,4]) and four-electron oxidized species ([5,5]); spectroscopic analysis of reaction products formed upon mixing the [3,3] and [5,5] ions indicated that the third oxidation state is a two-electron oxidized species ([4,4]). The [5,5] ion underwent first-order decay to the [4,4] ion with a rate constant, k similar or equal to 9.5 x 10(-3) s(-1), that was nearly identical with the catalytic turnover rate for O-2 evolution, k(cat) similar or equal to 1.3 x 10(-2) s(-1) measured under comparable conditions. The [4,4] ion underwent degradation more slowly to the [3,4] ion, which was stable on these time sc ales. An analogue bearing 4,4'-dimethyl-2,2'-bipyridine ligands exhibited very similar behavior, except that the oxidation steps were shifted by similar to 50 mV to lower potentials. O-18 isotope labeling experiments on the underivatized complex established that there was no oxygen exchange at the bridging mu-oxo position during catalytic turnover. Frozen solutions of the [5,5] ion displayed unusual low-temperature spectroscopic features, including the following: (i) a narrow g = 2.02 axial EPR signal exhibiting an apparent six-line hyperfine interaction from a minor component; (ii) a concentration-dependent broad rhombic EPR signal in mixtures also containing the [4,4] ion; and (iii) a concentration-dependent replacement of its characteristic ruthenyl Ru=O stretching mode at 818 cm(-1) in the RR spectrum when chemically oxidized with Ce4+ by an O-18 isotope sensitive set of three bands in the 650 cm(-1) region. The RR spectrum of this new species is consistent with further coordination of the terminal oxo ligands by Ce3+ to form additional ligand bridges.