We use molecular dynamics to predict the ionic conductivities of lithiated Nafion perfluorinated ionomeric membranes swelled in dimethyl sulfoxide (DMSO) and acetonitrile (ACN). The experimental conductivity of lithiated Nafion swollen with DMSO is two orders of magnitude higher than with ACN. Conversely, the mobility of Li+ ions in a solution of LiPF6 in ACN is approximately six times higher than in DMSO. In this work, we demonstrate that the ionic conductivity of Nafion is substantially governed by the concentration of free Li+ ions, i.e. by the degree of dissociation of the Li+ and SO3- pairs, and that the inherent mobility of Li+ in different solvents is of secondary importance. (C) The Author(s) 2016. Published by ECS. All rights reserved.