Abstract We present a study of perfluorinated ionomers with pendent chains terminated by sulfonic groups (Nafion), partially neutralized by paramagnetic VO 2+ cations. Electron spin resonance (e.s.r.) spectra of VO 2+ were measured in the temperature range 120–300 K in ionomer solutions and membranes swollen by water, methanol and ethanol, and compared with the spectra of VO 2+ in the neat solvents. E.s.r. spectra for all systems in the entire temperature range were simulated, using the Brownian diffusion motional model. The rotational correlation time τ c of the cation at 300K in the membranes swollen by water, in ionomer solutions in water and in neat water are all similar, and suggest that the cation is located in large water ‘pools’. This conclusion is in agreement with the expected phase separation of the ionomer into polar and non-polar domains that leads to a reverse micellar structure. For the alcohols, however, the e.s.r. spectra and corresponding τ c values of the cation in the swollen membrane and in ionomer solutions are similar, but clearly different from those measured in the neat solvents; these results were explained by assuming that the alcohols penetrate into the perfluorinated regions, and form small solvent pools with a diameter <20 Aå, where the motion of the cation is highly hindered.