Oxygen isotope fractionation is applied for the first time to probe the catalytic oxidation of water using a widely studied ruthenium complex, [Ru-II(tpy)(bpy)(H2O)](ClO4)(2) (bpy = 2,2'-bipyridine; tpy = 2,2';6 '',2 ''-terpyridine). Competitive oxygen-18 kinetic isotope effects (O-18 KIEs) derived from the ratio of O-16,16(2) to O-16,18(2) formed from natural-abundance water vary from 1.0132 +/- 0.0005 to 1.0312 +/- 0.0004. Experiments were conducted with cerium(IV) salts at low pH and a photogeneratect ruthenium(III) tris(bipyridine) complex at neutral pH as the oxidants. The results are interpreted within the context of catalytic mechanisms using an adiabatic formalism to ensure the highest barriers for electron-transfer and proton-coupled electron-transfer steps. In view of these contributions, O-O bond formation is predicted to be irreversible and turnover-limiting. The reaction with the largest O-18 KIE exhibits the greatest degree of O-O coupling in the transition state. Smaller O-18 KIEs are observed due to multiple rate-limiting steps or transition-state structures which do not involve significant O-O motion. These findings provide benchmarks for systematizing mechanisms of O-O bond formation, the critical step in water oxidation by natural and synthetic catalysts. In addition, the measurements introduce a new tool for calibrating computational studies using relevant experimental data.