Surface treatment of ruthenium dye (Bu(4)N)(2)[Ru(dcbpyH)(2)(NCS)(2)]-sensitized TiO(2) films using diethyl 4-methylphenylphosphonate as a coadsorbent results in large improvements in DSC performance, 24% larger overall conversion efficiency and 55 mV higher open-circuit voltage without current penalty. Introducing another two coadsorbents ethyl 4-methylphenylphosphonate and 4-methoxyphenylphosphonic acid brings in negative effects. Analysis by impedance spectroscopy indicated that this was attributed to different numbers of proton-releasing hydroxyl groups of the coadsorbents and consequently different proton-adsorbing chemical environments on the surface of TiO(2) photoanodes. (C) 2010 Elsevier B.V. All rights reserved.