Significance Development of rapid, robust water oxidation catalysts remains an essential element in solar water splitting by artificial photosynthesis. We report here dramatic rate enhancements with added buffer bases for a robust Ru(II) polypyridyl catalyst with a calculated half-time for water oxidation of ∼7 μs in 1.0 M phosphate. The results of detailed kinetic studies provide insight into the water oxidation mechanism and an important role for added buffer bases in accelerating water oxidation by concerted atom–proton transfer.