Abstract Complex impedance studies have been carried out in acid form Nafion 117 treated with various amounts of methanol and methanol–water mixtures. At room temperature and atmospheric pressure the conductivity for Nafion treated with “pure” methanol is about a factor of ten less than for Nafion which contains the same wt.% of water. In samples treated with the water–methanol mixtures, the conductivity is lower than for samples having the same total wt.% of water. However, for low mixed fluid wt.% the conductivity is significantly higher than for samples with the same amount of water, only, as was in the mix. This enhancement of conductivity over that for the corresponding water uptake is attributed to a plasticizing effect of the methanol facilitating the segmental motion of the polymer. At higher water concentrations, the conductivity is generally lower in the mixed solution-treated samples than in samples treated with the corresponding amount of water. This is to be expected since in this regime, proton conduction occurs in fluid-rich regions, which in the solution case includes a large fraction of methanol. For a 40 wt.% 1.4:1 molar ratio film, the studies were carried out at pressures up to 0.3 GPa. It is found that the electrical conductivity decreases with increasing pressure. Both the electrical conductivity and the activation volume are similar to the result for Nafion containing the same amount of water only. Deuteron NMR spin-lattice relaxation measurements of isotopically enriched methanol/water mixtures in Nafion 117 at elevated pressure demonstrate greater molecular-level interactions between methanol and Nafion than between water and Nafion. This is consistent with the plasticizing effect observed in the conductivity results.