Neutron spin-echo spectroscopy has been used to measure the time-scale of fluctuations associated with the inter-chain correlation peak observed in the diffraction pattern of the perfluorosulfonate ionomer, Nafion((R)). We have successfully measured the chain dynamics as a function of water content and temperature and have demonstrated that the chain dynamics become faster with increasing temperature. The addition of water also results in shorter relaxation times due to a plasticization of the chain motions. At water contents above a approximate to 6, the chain dynamics seem to approach a plateau and the relaxation times of the chains are no longer being plasticized by the presence of additional water molecules. The increased mobility in molecular relaxations induced by the presence of water points to the molecular origins of the temperature- and humidity-dependent softening mechanisms in Nafion and other perfluorinated sulfonic acid membrane materials. Moreover, these results show that there is a strong inter-dependence of the dynamics of water and the polymer chains in Nafion membranes. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. 2014, 52, 624-632