The influence of a polar plasticizer glycerol, and a nonpolar plasticizer, dioctyphthalate (DOP), on the microstructure and relaxation properties of poly(methyl methacrylate) ionomers has been investigated by dynamic mechanical thermal analysis. The test results indicate that glycerol strongly interacts with and weakens the ionic cluster ’phase’, and also significantly increases the mobility of backbone hydrocarbon chains in the multiplet-containing matrix phase. In contrast, the nonpolar plasticizer DOP is more selective in that it appreciably reduces the glass transition temperature of the hydrocarbon-rich matrix phase, but has a much smaller effect than glycerol on the glass transition temperature of the ion-rich cluster ’phase’. The present results are compared and contrasted with the effects of the same two plasticizers on polystyrene ionomers.