Platinum electrodes covered with a perfluoro-sulfonated ionomer membrane show unique behaviors in comparison with bare platinum immersed in aqueous solutions. The electrochemical interface between the metal and the polymer can be different from the interface between the metal and the solution phase. In this context, platinum electrodes covered with such a polymer membrane are an interesting system, and deserve detailed study. Here the oxygen reduction reaction at the platinum surface covered with a perfluoro-sulfonated ionomer membrane is investigated kinetically in 0.1 N H2SO4 in the presence of 0.001 N Na+, K+, or Ca2+ ions, using rotating disk electrodes. It is discovered that the impurity ions, even in small amounts, kinder enormously the rate of the charge transfer step of oxygen reduction at the platinum covered with perfluoro-sulfonated ionomer. Especially the effect of Na+ in the membrane is very serious considering the fact that there exists only 2% Na+ of the exchange site in the membrane phase at this condition. Platinum covered with perfluoro-sulfonated ionomer membrane has historically attracted much research interest, based on the fact that oxygen concentration and H+ ion concentration in the membrane are both larger than those in normal acidic solutions, and could show larger catalytic activity than in bare platinum (Gottesfeld, S.; et al. J. Electrochem. Sec. 1987, 134, 1455. Lawson, D. R.; et al. J. Electrochem. Sec. 1988, 135, 2247). However, this expectation encountered disappointing failure (Zecevic, S. K.; et al. J. Electrochem. Sec. 1997, 144, 2973). Results here indicate that such a paradox could be accounted for by the effect of the metal-polymer interface that alters the reaction conditions of oxygen reduction.