The dynamic mechanical properties and morphology of carboxylated polysulfone ionomers were investigated by dynamic mechanical thermal analysis and small-angle X-ray scattering (SAXS) techniques. It was found that at 25 mol % of ions, ionomers show two glass transitions: one at about 200 °C (the matrix T g ) and the other at about 235 °C (the cluster T g ). It was also found that with increasing ion content up to about 37 mol %, the matrix T g shifted to higher temperatures and the size of tan ␦ peak decreased. The cluster T g did not change. From the results, it is suggested that even at high ion content, the ionomers contain a significant amount of unclustered material, but that the increase in the ion content does not increase the amount of clustered material. SAXS profiles showed the ionic peak, which represents the presence of multiplets in the cluster regions. In addition, the difference in the matrix and cluster T g 's of this ionomer system was found to be about 35°. Thus, it is postulated that ionic group aggregation is subject to steric hindrance owing to the bulkiness of benzene ring, and tension on polymer chains surrounding the multiplet owing to chain rigidity, which limit the size and stability of the multiplet significantly.