The dynamic mechanical properties of a new ionomer system, poly(methyl methacrylate-co-sodium acrylate), were studied. In addition, to investigate the effect of the chemical structure of ionic group on the mechanical properties, the data obtained from the sodium acrylate ionomers were compared to those obtained from sodium methacrylate ionomers. The matrix and cluster T-gS for the methacrylates were found to be higher than those for the acrylates. It was argued that the difference in the T-gS might be explained with the concept of contact surface area of the chain.