A new type of amorphous ionomer based on poly(methyl methacrylate) (PMMA) was prepared. Dynamic mechanical thermal analysis (DMTA), coupled with some differential scanning calorimetry (DSC), was used to study the morphology of the PMMA-based ionomers. Results obtained are compared with those determined for poly(styrene-co-metal methacrylate) ionomers and are discussed in terms of multiplet-cluster models. Characteristic behavior common to amorphous ionomers was observed for the PMMA-based ionomers : two primary relaxations appeared for ionomers with high ion contents, with glass transition temperatures (T-g) increasing with ion content and the rubbery modulus increasing significantly. Within the ion content range studied, the matrix T-g increased at a rate of ca. 6 degrees C/(mol %), in contrast to ca. 3 degrees C/(mol %) for the PS ionomer. At a critical ion content of about 12 mol %, an ion-rich cluster phase dominates over the ionic multiplet-containing (matrix) phase. It was also found that the secondary (beta)-transition peak, reflecting the local motion of the side groups of PMMA, was spread into two in PMMA ionomers, which reflect two different environments corresponding presumably to the cluster phase and the matrix phase. Both of these secondary relaxation peak positions increased with ion content. It was observed that the divalent counterion calcium created stronger intermolecular interactions than the monovalent counterion sodium.