The proton transport mechanism in fully hydrated Nafion 117 membranes was examined via electrochemical impedance spectroscopy (EIS) and steady-state current-potential measurements both in a symmetric H-2, Pt vertical bar Nafion vertical bar Pt, H-2 cell and in a H-2, Pt vertical bar Nafion vertical bar Pt, air PEM fuel cell with hydrogen partial pressure values, P-H2, varied between 0.5 kPa and 100 kPa. In agreement with recent studies it is found that for low PH, values the steady-state current-potential curves exhibit bistability and regions of positive slope. In these regions the Nyquist plots are found to exhibit negative real part impedance with a large imaginary component, while the Bode plots show a pronounced negative phase shift. These observations are consistent with the mechanism involving two parallel routes of proton conduction in fully hydrated Nafion membranes, one due to proton migration in the aqueous phase, the other due to proton transfer, probably involving tunneling, between adjacent sulfonate groups in narrow pores. The former mechanism dominates at high P-H2 values and the latter dominates in the low P-H2 region where the real part of the impedance is negative. (C) 2011 Elsevier Ltd. All rights reserved.