Aqueous solutions of Nafion, an ionomer consisting of a perfluorinated backbone and pendant sulfonic groups, were studied by the electron spin resonance (ESR) spin probe method and by fluorescence spectroscopy. The spin probes us ed were 4-(N,N-dimethyl-N-alkyl)ammonium-2,2,6,6-tetramethylpiperidine-1-oxyl cations (CATn) with n, the number of carbon atoms in the alkyl group of the probe, equal to 1 (CAT1), 8 (CAT8), and 16 (CAT16). The ESR spectra of the probes were analyzed in terms of line shapes as a reflection of the local dynamics and N-14 hyperfine splittings as indicators of the local polarity. The line shapes point to slower dynamics for the larger probes, CAT8 and CAT16, suggesting that these probes are located deeper inside the aggregates compared to CAT1. The high local polarity reflected in the magnetic parameters was explained by assuming water penetration into the aggregates. The local polarity was estimated from the intensity ratio, R, of the third to the first vibronic peaks (R = I-III/I-I) in the fluorescence spectrum of pyrene (P) as a probe; R is the polarity index. In the Nafion micelles R = 0.64, indicating a polar local environment, in accord with the ESR results. By comparison, R is 0.53 in water, 0.75 in methanol, and 1.91 in a perfluorinated oil. The fluorescence data also suggest that the upper limit of the critical micelle concentration, cmc, for Nafion is 0.01% w/w. The local viscosity was estimated to be 74 cP at ambient temperature from the fluorescence spectrum of 1,3-bis(1-pyrene)propane (P3P), based on a previously determined calibration curve of the intensity ratio of the excimer-to-monomer emissions, I-E/I-M, VS Viscosity for 14 nonaqueous solvents of known viscosities.