Electron spin resonance (ESR) spectroscopy has been used to obtain information on chain mobility in spin-doped poly[styrene-co-(sodium methacrylate)] random ionomers. The results show that spin probe molecules randomly dispersed in ionomers respond to the matrix glass transition. However, when the probe molecules are anchored to the ionic multiplets, their mobility is reduced, indicating that the polymer segments adjacent to the ionic multiplets are constrained, and their mobility is restricted. Significant motions of the probe molecules start only when the kinetics of the ion-hopping process becomes relevant. Plasticization of the ionomers with polystyrene oligomers allowed for the simultaneous observation of two different mobility regimes, assigned to the matrix and cluster phases. These findings support the suggestion, which is a major component of the EHM model for the morphology of random ionomers, that regions of restricted mobility exist around the ionic multiplets.