The effect of neutralization level on the dynamic mechanical properties and morphology of partly neutralized poly(ethyl acrylate-co-Na acrylate) EANa ionomers was investigated. The mechanical data of EANa ionomers were compared to those of polystyrene-based ionomers having higher T (g) but lower ion content than the EANa ionomers. It was found that at ca. 44 % neutralization, compared to ca. 50 % neutralization for styrene ionomers, the ionomers started to show cluster T (g) and ionic modulus. Thus, it was suggested that the 40-50 % neutralization level might be a starting point, at which the amorphous Na-neutralized ionomers, regardless of the matrix T (g) and acid content, behaved like bi-phasic materials. The SAXS study suggested that with increasing neutralization level the number of multiplets increased slightly but the number of ion pairs per multiplet increased systematically.