We studied the isothermal hydrogen adsorption and reaction at the carbon and platinum (Pt) surface of the humidified E-TEK electrode of a polymer electrolyte fuel cell (PEMFC) with a Nafion (R) 117 membrane by impedance spectroscopy at 30 and 50 degrees C. We found three rate-limiting steps, one of them is interpreted as proton hydration, a step that has not been reported previously. We analysed the influence of water transport on the impedance and found that the semicircle for the charge transfer reaction and that of proton hydration are superimposed at 50 degrees C, but separate at 30 degrees C. Additional evidence is presented for hydrogen adsorption to the carbon surface prior to charge separation. The surface diffusion coefficient of hydrogen was calculated to 1 x 10(-7) m(2)/s. (c) 2006 Elsevier B.V. All rights reserved.