Mechanistic proposals to account for the reactivity of water-oxidizing ruthenium diimine complexes have often invoked participation of covalently hydrated or pseudobase intermediates formed by reaction of solvent with the polypyridyl ligands. Probing for these intermediates has proven difficult because the concentrations of detectable reactive species are very low under commonly used experimental conditions. However, we have recently found that these transients accumulate in photocatalytic oxidation systems at neutral pH. In this work, we show that the reaction rates of these transient species correlate with catalytic activity and, therefore, that they meet minimal kinetic criteria to be true reaction intermediates.