The proton dynamics in Nafion (R) 117 is investigated by comparison of the diffusion coefficient D-NMR estimated from PFG-NMR with that of D-sigma estimated from electrical conductivity. At high water content region, D-sigma is about two times higher than D-NMR as a result of Grotthuss mechanism. At low water content region, D-sigma and D-NMR are in good agreement with each other. Both of the diffusion coefficients decrease steeply at low water content region. It can be explained as a result of the percolation transition due to the isolation of ion clusters, which is suggested by the recent small angle X-ray scattering data. (C) 2009 Published by Elsevier B.V.