Ce(IV) oxidation ofthe ruthenium blue dimer leads to catalytic oxidation of water.Kinetic and spectroscopic studies have revealed evidence forRuVORuV as the active catalyst. Redoxcycling occurs between RuIIIORuIII andRuVORuV via a complex sequence of coupledsteps involving oxidations by Ce(IV) and second-order crossreactions of the dimer. The oxygen-evolving step is not ratedetermining in the catalyticcycle.