The possible design of an artificial solar fuel production system can be inspired by natural photosynthesis in which light-induced charge separation is coupled to redox reactions driven by molecular catalysts. The water oxidation reaction facilitated by catalysts represents an important and very challenging process required for the successful realization of artificial photosynthetic schemes. Ruthenium based water oxidation catalysts incorporating two metal centers have been known to catalyze water oxidation in the presence of sacrificial electron acceptors or driven by electrode bias according to a four-electron oxidation scheme, however a few intriguing exceptions have been reported where mononuclear Ru complexes can act as catalysts. We have studied the reactivity of mononuclear Ru polypyridyl complexes that are capable of catalytically splitting water in the presence of sacrificial electron acceptors. Product analysis during the course of the water oxidation reaction in the presence of Ce(IV) salts indicates that the integrity of some mononuclear catalysts is compromised as evidenced by carbon dioxide evolution, while others are stable under experimental conditions. The mechanistic details of water oxidation catalysis were derived from the kinetic behavior of reaction intermediates observed during time-resoled pulse radiolysis experiments and pH dependent electrochemical studies. The electronic properties of observed intermediates were analyzed using DFT and TD-DFT methods, and agree well with experimental data. The experimental and theoretical characterization of mononuclear Ru complexes in higher oxidation states points towards a significant radical character of the transient species that plays an important role in the mechanism of the water oxidation reaction.