The microphase structure of ionomers based on an amorphous, maleated ethylene-propylene copolymer was investigated by using small-angle X-ray scattering (SAXS) and solid-state NMR experiments. It was shown that in this kind of ionomers grafted maleic anhydride, its salts with Zn 2+ and a fraction of EPM chain fragments form immobilized, ion-rich aggregates. Three types of EPM chain units with different mobility were detected in the ionomers and in the ionomer precursor, which can be attributed to chain units with low mobility forming aggregates surrounded by an interfacial layer, EPM network chains interconnecting these aggregates, and network imperfections such as dangling ends and chain loops. When the the degree of neutralization is increased, the average dimension of the immobilized aggregate remains almost constant, while the thickness of the interfacial layer with restricted mobility slightly increases. The size of the aggregates in MAn-g-EPM ionomers is significantly larger compared to other ionomers, and as a consequence, the number of acid groups within an aggregate is also larger. At 50% neutralization, the number of aggregates suddenly decreases. The changes in some macroscopic properties, such as compression set and tensile properties, are related to the morphology of the ionomers as determined by SAXS and NMR. When the degree of neutralization is increased, the properties of the materials change due to strengthening of the ionic aggregates.