The mechanism of how impurity cations suppress the kinetics of oxygen reduction reaction on platinum surface covered with perfluoro-sulfonated ionomer film was studied in 0.1 N H2SO4 in the presence of various kinds of impurity ions of several concentrations. Impurity cations tested were Li+, Na+, K+, Ca2+, Fe3+, Ni2+, and Cu2+ with the amount of 0.1%, 1%, and 10% as compared with H+ in the solution. Platinum disk of a rotating disk electrode was spin-coated with Nafion solution, and after drying the Nafion film-covered platinum was obtained. The electrochemical measurements were performed to evaluate both charge transfer and diffusion kinetics of oxygen reduction at the Nafion film-covered electrode. It was discovered that the impurity ions hindered enormously the rate of charge-transfer step at platinum covered with perfluoro-sulfonated ionomer. The suppression started already at 0.1% level of impurity concentration, but did not increase much at over 1% level. No suppression effect for oxygen reduction was observed for a bare platinum in the solution containing impurity ions, indicating that the effect is specific to the metal electrode-ionomer membrane interface. Also both the diffusion coefficient of oxygen and oxygen concentration in the membrane decreased by the presence of impurity cations. It was implied that all the process is related to the reorientation of polymer networks in the membrane, which might bring about the modification of electric double layer at the platinum-ionomer interface.