The reasons of unusually large differences observed in photocurrent efficiencies for the oxidation of various organic and inorganic substrates at nanostructured TiO 2 photoelectrodes are discussed. The “redox cycling”, where a product of the hole transfer acts, in turn, as scavenger for the photogenerated electrons, appears as a frequent cause of weak photocurrents. Such a recombination mechanism operates not only under open-circuit conditions but also in the presence of large anodic bias applied to the conducting support of the TiO 2 electrode. Experiments conducted in the presence of both, an efficient hole scavenger, formate ion, and an electron acceptor, methyl viologen dication, showed that, in a major part of the anodically polarized nanostructured TiO 2 film, the quasi-Fermi level of electrons remains actually close to the conduction band edge potential. Importantly, addition to the solution of an electron acceptor causes large drop of the photocurrent both under weak and intense UV illumination generated by an argon-ion laser. On the other hand, a large number of organic molecules undergoing essentially irreversible photooxidation (e.g., to form CO 2 and H 2O) generate high photocurrents at the nanostructured TiO 2 electrodes. Such reactions, when occurring in the presence of large concentrations of the reactants, may involve direct hole transfer.