Ionic conductivity and swelling data are measured for Nafion(R) perfluorinated ionomeric membranes in nonaqueous solvents and solvent mixtures and correlated with solvent physical properties. The dependence of ionic conductivity on solvent uptake and cation type is examined for Nafion(R) 117 membranes with a nominal equivalent weight of 1100 g/eq, The most important factors determining ionic conductivity in membranes swollen with polar nonaqueous solvents are the solvent viscosity, molar volume, donor properties, and the solvent uptake by the membrane. Ionic conductivity is generally limited by dissociation of the cation from the fixed anion site indicating that the ionomer fixed anion site basicity is the critical membrane property. Means for increasing membrane ionic conductivity are discussed.