The compound Ru(bpy)(2)(dppz-R)(PF6)(2), where bpy is 2,2'-bipyridine and dppz-R is 11-(diethoxyphosphorylmethyl) dipyrido[3,2-a:2', 3'-c] phenazine, was prepared and anchored to mesoporous nanocrystalline (anatase) TiO2 thin films as a probe of the effects of interfacial water on excited-state charge transfer processes at semiconductor interfaces. In nitrogen-saturated fluid acetonitrile, the Ru(bpy)(2)(dppz-R)(PF6)(2) compound was found to be highly photoluminescent. Water was found to quench the excited state by a mechanism adequately described by the Perrin model, from which the radius of quenching was abstracted, 75 +/- 2 angstrom. The Ru(bpy)(2)(dppz-R)(PF6)(2) compounds were found to bind to the TiO2 thin films in high surface coverages, 5 x 10(-8) mol cm(-2). When these films were immersed in acetonitrile, long-lived excited states (tau = 825 ns) that were quenched by the addition of water were observed. About 30% of the excited states could not be quenched by water. Efficient electron injection, phi(inj) = 0.8, was observed after light excitation of Ru(bpy)(2)(dppz-R)/TiO2 in a 0.1 M LiClO4/acetonitrile solution. The addition of large concentrations of water, > 0.5 M, was found to decrease the injection yield to phi inj = 0.3.