In this work the oxygen reduction reaction and the transport of the reactant through the membrane were investigated at Pt/proton exchange membrane interfaces immersed in H2SO4 + H3PO4 mixtures using cyclic voltammetry, rotating-disk electrode techniques and potential-step chronoamperometry. Studies were carried out as a function of the electrolyte concentration and film thickness. A theoretical treatment for the interpretation of the chronoamperometric results of oxygen reduction on film electrodes is developed.