The exchange current density for the hydrogen oxidation/evolution reactions was determined in a proton exchange membrane fuel cell. Ultralow Pt-loaded electrodes (0.003 mg(Pt)/cm(2)) were used to obtain measurable kinetic overpotential signals (50 mV at 2 A/cm(2)). Using a simple Butler-Volmer equation, the exchange current density and transfer coefficient were determined to lie within the range of 235-600 mA/cm(Pt)(2) and 0.5-1, respectively. Due to the fast kinetics, no measurable voltage losses are predicted for pure-H-2/air proton exchange membrane fuel cell applications when lowering the anode Pt loadings from its current value of 0.4 mg(Pt)/cm(2) to the automotive target of 0.05 mg(Pt)/cm(2). (C) 2007 The Electrochemical Society.