The role of the non-linear conductivity of fully hydrated Nafion membranes is discussed on the performance of state-of-the-art PEM fuel cells. It is shown that the Nafion conductivity contains two components, one constant corresponding to proton transport in the aqueous phase of the membrane, the other exponentially dependent on potential, linearly increasing with membrane thickness and strongly increasing with hydrogen partial pressure at the anode. A simple macroscopic mathematical model, accounting for the observed exponential dependence of Nafion conductivity on potential is developed and shown to provide a semiquantitative fit to the experimental I-U curves. (c) 2006 Elsevier B.V. All rights reserved.