Electron spin resonance (ESR) spectroscopic measurements using nitroxide spin probes were performed to investigate the microphase-separated structure of sodium-neutralized poly(ethylene-ran-methacrylic acid) [E-MAA] ionomers as a function of temperature from 77 to 403 K. It was found that the probes selected are intercalated position-selectively into the ionomers, and information about the local chain mobility around those probes was successfully obtained from the extreme separation of the spectra. Their temperature variation indicated which site is changed when temperature is varied through three characteristic temperatures of the ionomers (glass transition of the backbones [T-g approximate to 230-240 K], ionic aggregate transition [T-i approximate to 320 K], and polyethylene lamellae melting [T-m approximate to 360 K]). The results demonstrated that the effect of an ionic aggregate on the chain mobility becomes larger and extends to a longer distance from the aggregate at the higher degrees of neutralization x; at x = 0.8 and 0.9, the spectra of spin probes located in the amorphous region showed the presence of two dynamical states, corresponding to the matrix and ionic cluster phases in the Eisenberg et al. terminology, and the restricted mobility of the chains in the cluster phase persisted beyond T-m. On the other hand, the spectra of the probes deeply intercalated into the aggregate core clearly changed at T-i for x greater than or equal to 0.6, accompanied by a characteristic thermal hysteresis similar to the behavior of the DSC T-i peak. This result clearly supports the model of an ionic aggregate transition proposed by us previously.