Electron spin resonance (ESR) spectroscopy based on nitroxide spin probes was applied to the study of self-assembling in poly(ethylene-co-methacrylic acid) ionomers in aqueous solutions and in the corresponding membranes swollen by water. The probes selected for study were based on the stearic acid backbone, differed in their hydrophobicity and in the position of the nitroxide group with respect to the head group, and are known to intercalate in the chain aggregates and to report on the hydration level at various depths from the ionomer-solvent surface. The dynamic properties of the aggregates were deduced from ESR spectra measured in the temperature range 120-360 K for the solutions and 120-410 K for the swollen and dry membranes and from thermal transitions measured by DSC. ESR results for the solutions indicate that a clear aggregate-solvent interface exists even at 360 K; inside the aggregate, however, the local mobility is fast on the time scale of the ESR experiment, and the correlation times of the probes are similar to 10(-9) s/rad. Restricted mobility was detected in a layer of thickness of similar to 10 Angstrom from the aggregate-solvent interface and was attributed to constraints arising from the presence of proximal ionic groups. In the case of membranes swollen by water, the ESR spectra of the spin probe located near the water pools changed dramatically near 335 K, and two components differing in their local mobility were detected. Possible reasons for this behavior were discussed. Taken together, the results obtained in this study demonstrate that the amphiphilic spin probes behave as "dipsticks" for the dynamics in the aggregates and provide details on the specific and separate effects on dynamics of the polar domains, the chain characteristics (glass transition of amorphous polyethylene and melting transition of crystalline domains), and the solvent.