An ITO (indium tin oxide) electrode modified with [Ru(terpy)-{4,4'-(CH2PO3H2)(2)-2,2'-bpy}(OH2)](2+) was prepared to gain insight into the mechanism of O-2 evolution from water catalyzed by a series of mononuclear aquaruthenium complexes, such as [Ru-(terpy)(bpy)(OH2)](2+) The modified ITO electrode showed a clear decrease in overpotential for O-2 evolution (1 35 V in an aqueous 0 1 M Na2SO4/H2SO4 solution, pH 4 0), indicating that the O-2 evolution proceeds in a ummolecular fashion without having any intermolecular associations