Earlier O-18-H2O labeling studies had indicated that two concurrent pathways may exist for water oxidation catalyzed by [Ru(bpy)(2)(OH2)](2)O4+, a mu-oxo bridged diruthenium complex known colloquially as the "blue dimer". Specifically, the distribution of O-2 isotopomers obtained following its generation by the catalytically active form, [Ru(bpy)(2)(O)](2)(4+), suggested pathways in which either (1) one O atom was obtained from the terminally coordinated oxo atom and the second from the solvent or (2) both O atoms were obtained from the solvent. Plausible mechanisms have been advanced for the former pathway, but the second is enigmatic. In the present study, experiments are described that eliminate possibilities that the second pathway arises artifactually from rapid water exchange in reactive intermediary oxidation states of the catalyst, by mechanisms involving scrambling of the O-2 that is formed during reaction, or by mechanisms involving participation of the oxidant (Ce4+ or S2O82-). Comparative studies of partitioning between the two pathways made using catalysts containing substituted bipyridine ligands are consistent with a previously proposed pathway that involves noninnocent participation of these ligands.