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High ionic conductivities of 10-4 to 10-2 S/cm are achieved with Li+-form perfluorosulfonate ionomers over a wide temperature range by swelling in nonaqueous organic solvents. The dependence of ionic conductivity on temperature, solvent absorption, and membrane equivalent weight is examined for Nafion perfluorinated ionomer membranes. These results are compared with other ionomer membranes, including those having hydrocarbon backbones and weaker acid groups, to correlate ionic conductivity with ionomer structure. The most important factors determining ionic conductivity in membranes swollen with polar nonaqueous solvents, beyond the solvent properties such as viscosity and molecular weight, are the basicity of the fixed anion group and the solvent uptake by the membrane. Ionic conductivity is generally limited by dissociation of the cation from the fixed anion site. Several means for increasing conductivity are demonstrated including the use of cation complexing agents to increase ionic dissociation.