The mechanism of the catalytic oxidation of water by cis, cis-[(bpy)(2)Ru(OH2)](2)O4+ to give molecular dioxygen was investigated using Density Functional Theory ( DFT). A series of four oxidation and four deprotonation events generate the catalytically competent species cis, cis-[(bpy)(2)(RuO)-O-V](2)O4+, which breaks the H-OH bond homolytically at the rate determining transition state to give a hydroperoxo intermediate. Our calculations predict a rate determining activation barrier of 25.9 kcal/mol in solution phase, which is in reasonable agreement with the previously reported experimental estimate of 18.7-23.3 kcal/mol. A number of plausible coupling schemes of the two metal sites including strong coupling, weak ferromagnetic and weak antiferromagnetic coupling have been considered. In addition, both high-spin and low-spin states at each of the Ru(V)-d(3) centers were explored and we found that the high-spin states play an important mechanistic role. Our calculations suggest that cis, cis-[(bpy)(2)(RuO)-O-V](2)O4+ performs formally an intramolecular ligand-to-metal charge transfer when reacting with water to formally give a cis, cis-[(bpy)(2)(RuO)-O-IV center dot](2)O4+ complex. We propose that the key characteristic of the diruthenium catalyst that allows it to accomplish the most difficult first two oxidations of the overall four-electron redox reaction is directly associated with this in situ generation of two radicaloid oxo moieties that promote the water splitting reaction. A proton coupled metal-to-metal charge transfer follows to yield a Ru(V)/Ru(III) peroxo/aqua mixed valence complex, which performs the third redox reaction to give the superoxo/aqua complex. Finally, intersystem crossing to a ferromagnetically coupled Ru(IV)/Ru(III) superoxo/aqua species is predicted, which will then promote the last redox event to release triplet dioxygen as the final product. A number of key features of the computed mechanism are explored in detail to derive a conceptual understanding of the catalytic mechanism.