Electrochemical reduction of oxygen was studied at the epitaxially grown ultra thin Pd layers of various thicknesses on Au(111) and Au(100) single crystal surfaces in 50 mM HClO4 solution saturated by oxygen. The oxygen reduction at the Pd thin layers on the gold substrate started at more positive potential than that at the gold single crystal surfaces even only a submonolayer of Pd was deposited. The potentials where cathodic current of oxygen reduction started to increase were dependent on the potentials where the oxide of the ultra thin Pd layers was reduced.