The cis, cis-[(bpy)(2)Ru-III(OH2)](2)O4+ mu-oxo dimeric coordination complex is an efficient catalyst for water oxidation by strong oxidants that proceeds via intermediary formation of cis,cis-[(bpy)(2)Ru-V(O)](2)O4+ (hereafter, {5,5}). Repetitive mass spectrometric measurement of the isotopic distribution of O-2 formed in reactions catalyzed by O-18-labeled catalyst established the existence of two reaction pathways characterized by products containing either one atom each from a ruthenyl O and solvent H2O or both O atoms from solvent molecules. The apparent activation parameters for mu-oxo ion-catalyzed water oxidation by Ce4+ and for {5,5} decay were nearly identical, with DeltaH(double dagger) = 7.6 (+/-1.2) kcal/mol, DeltaS(double dagger) = -43 (+/-4) cal/deg mol (23 degreesC) and DeltaH(double dagger) = 7.9 (+/-1.1) kcal/mol, DeltaS(double dagger) = -44 (+/-4) cal/deg mol, respectively, in 0.5 M CF3SO3H. An apparent solvent deuterium kinetic isotope effect (KIE) of 1.7 was measured for O-2 evolution at 23 degreesC; the corresponding KIE for {5,5} decay was 1.6. The O-32(2) /O-34(2) isotope distribution was also insensitive to solvent deuteration. On the basis of these results and previously established chemical properties of this class of compounds, mechanisms are proposed that feature as critical reaction steps H2O addition to the complex to form covalent hydrates. For the first pathway, the elements of H2O are added as OH and H to the adjacent terminal ruthenyl O atoms, and for the second pathway, OH is added to a bipyridine ring and H is added to one of the ruthenyl O atoms.