Higher oxidation states of the mu-oxo bridged ruthenium "blue dimer" ([Ru(bpy)(2)(OH2)](2)O4+) have been characterized by redox titration measurements, resonance Raman (RR) spectroscopy, EPR spectrometry, and pulse radiolysis. The cumulative results indicate that the progression of accessible oxidation states in acidic media is {3,3} -> {3,4} -> {4,4} -> {5,5}, but changes to {3,3} -> {3,4} -> {4,5} -> {5,5} above pH 2. Although the reaction 2{4,5} + 2H(2)O -> 2{3,4} + O-2 is thermodynamically favorable, no O-2 was detected during the decay of {4,5} to {3,4}. One-electron oxidation of {3,4} by radiolytically generated sulfate and carbonate radicals allowed determination of the {4,4} optical spectrum in neutral and alkaline media, where it exists only as a short-lived transient species. This spectrum was identical to that previously reported for {4,4} in acidic media; this observation and comparative RR spectra suggest that its molecular formula is [Ru(bpy)(2)(OH)](2)O4+, that is, both Ru atoms contain a coordinated hydroxo ligand. Upon application of an acidic pH jump, electrochemically prepared {4,5} underwent disproportionation to {4,4} and {5,5}, as determined from changes in the EPR spectra of the solutions. These studies clarify the nature of redox transients formed during water oxidation catalysis by the "blue dimer", thereby providing information that is critical to performing accurate mechanistic analyses.