Structural and dynamical information on the self-assembling of perfluorinated ionomers (Nafion) in aqueous solutions was obtained by doping the solutions with nitroxide spin probes and measuring the electron spin resonance spectra as a function of polymer concentration and temperature. The probes are based on stearic acid, either as the lithium salt or as the methyl ester. Two spectral sites, assigned respectively to probes associated with large aggregates of low mobility and with faster and smaller objects, have been detected. The small objects are tentatively assigned to single chain micelles (unimers). Additional support for this assignment is obtained by surface tension measurements. The spectral parameters of the probes in both sites suggest a polar environment, indicating that the probes "drag" part of their solvation shell into the hydrophobic environment. The aggregate size increases when the ionomer concentration increases. The relative intensity of the component associated with the unimers increases with temperature, pointing to a temperature-dependent equilibrium between large aggregates and unimers. Analysis of the temperature variation of the signal associated with the large aggregates indicates different degrees of probe immobilization, the more hydrophobic probes reflecting more rigid and ordered regions of the aggregates. Only probes associated with large aggregates were detected in the membranes. The results obtained have been compared with small-angle scattering and F-19 NMR data. This study indicates the advantages and limitations of the nitroxide spin probe method, for the case of protiated probes in solutions of a perfluorinated polymer surfactant.