We synthesized 1,8-bis(2,2':6',2"-terpyrid-4'-yl)anthraquinone (btpyaq) as a new dimerizing ligand and determined its single crystal structure by X-ray analysis. The dinuclear Ruthenium complex [Ru-2(mu-Cl)(bpy)(2)(btpyaq)](BF4)(3) ([3](BF4)(3), bpy = 2,2'-bipyridine) was used as a catalyst for water oxidation to oxygen with (NH4)(2)[Ce(NO3)(6)] as the oxidant (turnover numbers = 248). The initial reaction rate of oxygen evolution was directly proportional to the concentration of the catalyst and independent of the oxidant concentration. The cyclic voltammogram of [3](BF4)(3) in water at pH 1.3 showed an irreversible catalytic current above +1.6 V (vs. SCE), with two quasi-reversible waves and one irreversible wave at E-1/2 = +0.62, +0.82 V, and E-pa = +1.13 V, respectively. UV-vis and Raman spectra of [3](BF4)(3) with controlled-potential electrolysis at +1.40 V revealed that [Ru(IV)=O O=Ru(IV)](4+) is stable under electrolysis conditions. [Ru(III), Ru(II)] species are recovered after dissociation of an oxygen molecule from the active species in the catalytic cycle. These results clearly indicate that an O-O bond is formed via [Ru(V)=O O=Ru(IV)](5+).