The kinetics of the reduction of oxygen on platinum covered by a Nafion((R)) film in sulfuric acid (0.5 M) has been studied in order to determine to what extent the solid polymer electrolyte modifies this reaction. As electrode we used a rotating electrode which is particularly well adapted to the measurement of the permeability DfCf (product of the diffusion coefficient and of the oxygen concentration in the film) of oxygen in the Nafion((R)) film. This product is of the order of 6 x 10(-12) M cm(-1) s(-1) whatever the state of division of the platinum, and is of the same order of magnitude as the permeability DsCs of oxygen in the adjacent sulfuric acid solution. It is shown, moreover, that the oxygen concentration in the film is very high, about five times that in the solution. (C) 2004 Elsevier B.V. All rights reserved.