The conductivity of fully hydrated Nalion 112, 1135, 115 and 117 membranes was measured via ac impedance spectroscopy and steady-state current-potential measurements both in symmetric H-2 Pt vertical bar Nafion vertical bar Pt, H-2 and D-2 Pt vertical bar Nafion vertical bar Pt, D-2 PEM cells and in H-2, Pt vertical bar Nafion vertical bar Pt, airand D-2, Pt vertical bar Nafion vertical bar Pt, air PEM fuel cells. In agreement with recent studies, it was found that the conductivity, sigma, increases almost linearly with membrane thickness L and also depends exponentially on potential and almost linearly on P-H2(1/2). These and other observations, including the strong isotope effect obtained upon switching between H-2 and D-2 at the anode, show that the conductivity of Nafion contains two components, one due to proton migration in the aqueous phase, the other due to proton tunneling between adjacent sulfonate groups in narrow pores. The observed near-linear increase of a with L is consistent with the proton tunneling mechanisin but can also be explained by the existence of skin layers with lower conductivity at the ionomer interfaces with the anode and cathode. (c) 2005 Elsevier Ltd. All rights reserved.