The mechanism of Ce(IV) water oxidation catalyzed by [Ru(tpy)(bpm)(OH2)](2+) (tpy = 2,2':6',2 ''-terpyridine; bpm = 2,2'-bipyrimidine) and related single-site catalysts has been determined by a combination of mixing and stopped-flow experiments with spectrophotometric monitoring. The mechanism features O---O coupling by water attack on Ru-V=O3+ and three peroxidic intermediates that have been characterized by a combination of spectroscopy and DFT calculations.