Dynamic mechanical properties of poly(methyl acrylate-co-sodium acrylate) ionomers were studied. It was found that the ＇ionic＇ modulus of the poly(methyl acrylate) (PMA) ionomers was lower than that for the poly(styrene-co-sodium methacrylate) (PS) ionomers at the same ion content. This implies that the PMA ionomer is less clustered than the PS ionomer, which is explained in terms of the differences in the glass transition temperature and polarity of the polymer matrix. It was also found that the increasing rates in the matrix and cluster T(g)s Of the PMA ionomers were similar to those of the PS ionomers. However, the difference in the matrix and cluster T(g)s was higher for the PS ionomer than for the PMA system because of the lower clustering of the PMA ionomers. (C) 2000 Elsevier Science Ltd. All rights reserved.