The phase-separated morphology and the effect of ionic aggregation on the backbone chain mobility in poly(ethylene-ran-methacrylic acid) (EMAA) ionomers neutralized by Na+ were investigated as a function of the degree of neutralization, x, by spin-probe electron spin resonance (ESR) spectroscopy and small-angle X-ray scattering (SAXS). The shape and size of the aggregates and the number of ionic groups (anions and counterions) in each aggregate were determined by SAXS, which showed that the ionic core of the aggregate is a sphere with radius of approximate to6 Angstrom that contains 7 - 12 ionic groups; in addition, the results strongly suggested that the sphere is covered by a shell of chain segments whose electron density is slightly lower than that of the ionomer matrix. The ESR spectra of five nitroxide spin probes differing in their hydrophilicity and in the position of the nitroxide group with respect to the headgroup were analyzed. The results indicated that the probes are position-selective and can provide information on the local polarity and mobility in, and near, the ionic aggregates. The effect of ionic aggregation on the backbone chain mobility was determined from the analysis of the ESR spectra of the spin probes at ambient temperature and in the "rigid limit" (77 K); the spectra clearly indicated the highly restricted mobility of the chain segments in the hydrocarbon shell surrounding the ionic aggregates. This study showed the complementarity of SAXS and ESR spectroscopy for deducing the phase structure and dynamics in ionomers.