The influence of molecular side groups on the electron backtransfer into ruthenium-dye molecules adsorbed on ultra-thin nanoporous TiO2 layers has been investigated by modulated surface photovoltage spectroscopy in argon atmosphere. All ruthenium-dye molecules contained one bipyridine (bpy) ligand with two carboxylic groups for anchoring at the TiO2 surface. Ruthenium (II) complexes with a second bpy ligand with branching and non-branching side groups containing C and H only and ruthenium (II) complexes with a second pyridine (py) ligand with side groups containing different numbers of -C-O-C- sequences were synthesized. The electron backtransfer was practically not affected by branching or non-branching side groups in the bpy-bpy complexes. In contrast, electron backtransfer was much less for bpy-py complexes in comparison to bpy-bpy complexes and increased strongly with increasing number of -C-O-C- sequences in the side groups. (C) 2009 Elsevier B.V. All rights reserved.