Aqueous solutions of the ionomer poly(ethylene-co-methacrylic acid) (EMAA) in the concentration range 0.5-23 wt % were studied by the electron spin resonance (ESR) spin probe method. The spin probes selected for this study were 4-(N,N-dimethyl-N-alkyl)ammonium-2,2,6,6 -tetramethylpiperidine-1-oxyl iodides (CATn) with different alkyl groups : methyl (CAT1), octyl (CAT8), and hexadecyl (CAT16). The main objective of this study was to compare the ESR spectra of the CATn probes as a function of temperature and ionomer content in order to deduce details on the structure and dynamics of EMAA micelles and on the interface region between the aggregates and the solvent. ESR spectra were recorded in the temperature range 120-360 K, and the line shape variations and the N-14 hyperfine splittings, alpha(N), were key parameters fop the interpretation of the results in terms of the spin probe mobility, and its＇ location in and bonding to the polymeric aggregates. The detection of two spectral components in the ESR spectra of the probes provided evidence for the existence of an equilibrium between large multichain aggregates and unimeric micelles, in accord with previous ESR (based on doxylstearic acid spin probes) and fluorescence (based on pyrene as the luminophore) studies. The results indicated that most spin probe molecules are bound to large intermolecular micelles; the long alkyl chains of CAT8 and CAT16 penetrate into the interior of the aggregates but exhibit different dynamics, suggesting that the longer alkyl chain of CAT16 penetrates deeper, into the more viscous regions of the micelle, compared to CAT8. This conclusion implies that the cationic probes with different alkyl substituents can be used to map the local viscosity and the viscosity gradient. In the EMAA solutions an isotropic triplet was also detected, as a minor spectral component. Its contribution depends on the type of probe and on ionomer concentration and ranges from approximate to 1% for CAT16 in the 23 wt % EMAA solution to approximate to 10-15% (depending on probe concentration) for CAT8 in the 0.5 wt % EMAA solution at 300 K. This component was assigned to spin probes bound to unimeric (intramolecular) micelles. Analysis of the N-14 hyperfine splittings indicated the formation of an ionic bond between the carboxylic group of the ionomer and the cation of the probes.