Surface mesoscopic titanium dioxide P25 films deposited onto conducting glass plates SnO2 F were modified by colloidal RuxSey nanoparticles 2 nm diameter . A decrease of the photocurrent was found upon modification of TiO 2 films. However, interfacial electron transfer kinetics to oxygen was favored. The increase of the catalyst surface concentration onto TiO 2, shifts the onset of the photocurrent under UV illumination, to 0.6V RHE in presence of oxygen dissolved in the electrolyte. Concomitant to this, the cathodic current becomes important and shifts to more positive potentials. This phenomenon allows the system to work catalytically under open circuit conditions. On non modified TiO2, the application of a 0.3 V RHE potential leads to an enhancement of the photo oxidation of formic acid. Photocurrent images revealed a nonhomogeneous distribution of the catalyst on the titanium dioxide films