The synthesis, characterization, and redox properties are described for a new ruthenium-based chromophore catalyst assembly, [(bpy)(2)Ru(4-Mebpy-4'-bimpy)Ru(tpy)(OH2)](4+) (1, [Ru-a(II)-Ru-b(II)-OH2]4(+;) bpy = 2,2'-bipyridine; 4-Mebpy-4'-bimpy = 4-(methylbipyridin-4'-yl)-N-benzimid-N'-pyridine; tpy = 2,2':6',2 ''-terpyridine), as its chloride salt. The assembly incorporates both a visible light absorber and a catalyst for water oxidation. With added ceric ammonium nitrate (Ce-IV, or CAN), both 1 and 2, [Ru(tpy)(Mebim-py)(OH2)](2+) (Mebim-py = 2-pyridyl-N-methylbenzimidazole), catalyze water oxidation. Time-dependent UV/vis spectral monitoring following addition of 30 equiv of Ce-IV reveals that the rate of Ce-IV consumption is first order both in Ce-IV and in an oxidized form of the assembly. The rate-limiting step appears to arise from slow oxidation of this intermediate followed by rapid release of O-2. This is similar to isolated catalyst 2, with redox potentials comparable to the [-Ru-b-OH2](2+) site in 1, but 1 is more reactive than 2 by a factor of 8 due to a redox mediator effect.