The kinetics and mechanism of Ce-IV oxidation of the water oxidation catalyst [(bpy)(2)(H2O)Ru-III-ORuIII(OH2)(bpy)(2)](4+) (1, (RuORuIII)-O-III) have been investigated by UV-visible measurements with application of global analysis. The reaction proceeds by stepwise oxidation of (RuORuIII)-O-III to (RuORuV)-O-V with oxidation of (RuORuIII)-O-IV the slow step. (RuORuV)-O-V has been identified as an intermediate by the appearance of its ClO4- salt as a black suspension in concentrated solutions at 5 degrees C. It is the key intermediate in water oxidation. The mechanism may involve a bimolecular step and formation of a peroxo-bridged intermediate. Catalytic water oxidation is greatly retarded after just a few catalytic cycles because of anation induced by O-2 evolution.