The effects of the addition of LiCl and CsCl salts to Li- and Cs-neutralized styrene-co-methacrylate ionomers, respectively, on the mechanical properties and morphology of the ionomers were studied. It was observed that with increasing inorganic salt contents, the ionic modulus increased, and this indicated that the inorganic salts in the ionomers acted as fillers. However, the type of salt did not affect the increase in the ionic modulus. It was also found that the addition of the inorganic salts did not change the matrix glass-transition temperatures of the ionomers strongly but reduced the cluster glass-transition temperature significantly and slowly for the LiCl- and CsCl-containing ionomers, respectively. In addition, with increasing salt contents, a small-angle X-ray scattering peak shifted to slightly lower angles. These findings suggested that some of the inorganic salts resided in the multiplet with the ionic groups of the ionomers, acting as plasticizers. The presence of an X-ray diffraction peak for the polymers containing a relatively large amount of CsCl indicated that the CsCl salt formed phase-separated domains at sufficiently high salt contents. (C) 2008 Wiley Periodicals, Inc.