The oxygen reduction reaction (ORR) at platinum electrodes covered with perfluorosulfonated ionomer film was studied in 0.05 mol dm(-3) H2SO4 in the presence of various kinds of ammonium ions. Ammonium ion derivatives (R4N)(2)SO4 (R = H, CH3) and (R4N)HSO4 (R = C2H5, C3H7, and C4H9) were added with various amounts in the solution. The electrochemical measurements of ORR were performed to evaluate both charge transfer and diffusion kinetics of oxygen reduction at the Nafion film covered platinum rotating disk electrode. ORR was affected differently for different molecular weights of ammonium derivative ions. The suppression of ORR started in a short time for ammonium derivative ions of high molecular weight, reflecting the strong adsorption at the platinum surface. It was inferred that these ions strongly distorted the electric field at the platinum \ ionomer interface. From the knowledge, a new method to inhibit the ORR degradation at platinum \ ionomer interface was proposed, and several kinds of carboxylic acids or amino acids were tested as additives in the ionomer film. It was discovered that these additives successfully inhibit the ORR degradation brought about by impurity cations. After investigating the ORR kinetics, it was found that the additives prevented the degradation of the charge transfer step effectively, but the diffusion process in the film was degraded more by these additives.