Soluble perfluorinated ionomers (Nafion) were spin-labeled via the ionic interaction between the pendant sulfonic group of the polymer and the quaternary nitrogen of the deuteriated polar spin label. 4-(trimethylammonio)-2,2,6,6-tetramethylperdeuteriopiperidine-N-oxyl iodide. . Electron spin resonance (ESR) spectra of spin-labeled Nafion were measured in water, ethanol, formamide, and N-methylformamide solutions. Spin-labeled Nafion membranes saturated by water and aqueous solutions of spin-labeled poly(styrene sulfonic acid) as the sodium salt were also studied for reference. The ESR spectra suggest reduced mobility of the label in aqueous solutions of Nafion compared to that of the free spin label in the neat solvent. A smaller effect was observed for ethanol as solvent and practically no effect in formamide and N-methylformamide solutions. The results were rationalized by considering the specific influence of each solvent on the solvophobicity of the ionomer backbone and on the interaction between ion pairs. We suggest that the driving force for the self-assembling of perfluorinated ionomers in polar solvents is the solvophobicity of the ionomer chains.