Spectral response measurements have been performed on dye-sensitized nanoporous-nanocrystalline TiO2 photoelectrodes. The effects of film thickness, electron scavengers in the electrolyte, and surface treatment of the nanocrystalline film were studied by means of action spectra for front- and back-side illumination. Our results show that electron accepters such as dioxygen and iodine strongly decrease the IPCE. Surface treatment of the electrode with pyridine induces a substantial increase of the photocurrent yields. The observations are discussed in terms of kinetics at the semiconductor-electrolyte interface. IPCE values for sandwich cells were generally much higher than those obtained from three-electrode measurements.