The electrochemical reduction of oxygen on thin Pd films with a nominal thickness of 0.25-10 nm on polycrystalline Au substrate (Pd/Au) was studied. The Pd films were prepared by electron beam evaporation and oxygen reduction was studied in 0.1 M HClO4 and 0.05 M H2SO4 solutions using the rotating disk electrode (ROE) method. The surface morphology of Pd overlayers was examined by scanning tunnelling microscopy (STM). O-2 reduction predominantly proceeds through 4e(-) pathway on all Pd/Au electrodes. The specific activity (SA) of oxygen reduction was lower in H2SO4 solution and decreased slightly with decreasing the Pd film thickness. In HClO4, the SA was higher and not significantly dependent on the film thickness. The Tafel slope values close to -60 mV at low current densities and -120 mV at high current densities were found for all electrodes. (c) 2010 Elsevier Ltd. All rights reserved.