Dynamic mechanical properties and morphology of poly(styrene-co-sodium methacrylate) ionomers containing aromatic organic salts possessing one, two, and four sodium carboxylate ion pairs per salt molecule, i.e. Na-benzoate, Na-isophthalate, and Na-benzenetetracarboxylate, respectively, were investigated. The addition of the organic salts increased the ionic modulus noticeably and lowered the cluster T-g slightly. Thus, it was suggested that some of the organic salts might reside in/near multiplets, acting as plasticizer, while the rest of the salts formed aggregates, acting as filler. It was also found that the amount of the organic salts, not the number of ion pairs per one organic salt molecule, played a key role in the determination of the mechanical properties of the ionomers. It was also observed that the experimental ionic modulus data were much higher than the calculated values using filler concepts, which implied that the addition of salts also changed the nature of the polymer matrix significantly. (C) 2003 Elsevier Science Ltd. All rights reserved.