The paper presents electrochemical measurements carried out in a PEMFC with a view to determining the separate kinetics of the electrode reactions. For this purpose, the separate response of one electrode (anode or cathode) was magnified by dilution of the reacting gas, respectively hydrogen and oxygen, and comparison of the experimental data in the form of steady voltage-current variations and impedance spectra. Experiments were carried out at 60 degrees C and ambient pressure. Water management was thoroughly controlled so that the gases leaving the cell had the same relative humidity in all experiments of one series. Hydrogen oxidation, although rapid, corresponds to overpotentials up to 50 mV at high dilution rates and current densities. Assuming a Tafel-Volmer mechanism, the exchange current density of the anode reaction at the Pt surface is of the order of 1 mA cm(-2). The two techniques employed led to Tafel slopes of oxygen reduction ranging from 120 to 150 mV/decade, with an exchange current density near 1 mu A cm(-2), in good agreement with published data.