The photoelectrochem. properties of nanostructured highly porous TiO2 electrodes with different thicknesses (2 to 38 μm) were investigated. The effects of electron acceptors, such as oxygen and iodine, in the electrolyte were studied by action spectra measurements. The change due to oxygen and iodine are explained by increased loss of photogenerated electrons over the nanostructured semiconductor electrolyte interface during the transport of the electrons to the back contact. PH had a pronounced effect on the spectral distribution. At pH 1.2, the photoresponse is high and independent of the presence of oxygen. Front side illumination of a 33-μm-thick electrode measured in a stirred nitrogen purged soln. converts amazingly 10% of the photons to current indicating that the transport of electrons in these films can be extremely good when the electrolyte compn. is favorable. The efficiency for back side illumination increases with film thickness, even up to 38 μm, in stirred solns. The investigation shows that one must consider the electrode and the electrolyte in the cavities as an integrated nanostructured system. It also illustrates important parameters in three-electrode measurements on these kinds of systems.