Iridium oxide nanoparticles stabilized by a heteroleptic ruthenium tris(bipyridyl) dye were used as sensitizers in photoelectrochemical cells consisting of a nanocrystalline anatase anode and a Pt cathode. The dye coordinated the IrO(2) x nH(2)O nanoparticles through a malonate group and the porous TiO(2) electrode through phosphonate groups. Under visible illumination (lambda > 410 nm) in pH 5.75 aqueous buffer, oxygen was generated at anode potentials positive of -325 mV vs Ag/AgCl and hydrogen was generated at the cathode. The internal quantum yield for photocurrent generation was ca. 0.9%. Steady-state luminescence and time-resolved flash photolysis/transient absorbance experiments were done to measure the rates of forward and back electron transfer. The low quantum yield for overall water splitting in this system can be attributed to slow electron transfer (approximately 2.2 ms) from IrO(2) x nH(2)O to the oxidized dye. Forward electron transfer does not compete effectively with the back electron transfer reaction from TiO(2) to the oxidized dye, which occurred on a time scale of 0.37 ms.[on SciFinder (R)]