Deformation mechanisms of poly(methyl methacrylate) (PMMA)-based ionomers subject to simple tension were studied by transmission electron microscopy (TEM). The deformation mechanism changed from crazing only for PMMA to crazing plus shear deformation for the Na-PMMA ionomer with 6.0 mol% ion content and to shear deformation only for the Na-PMMA ionomer with 12.4 mol% ion content. Such changes with ion content are similar to those observed for polystyrene (PS)-based ionomers and can be understood as arising from an increased "effective" strand density due to the ionic cross-linking effect. It was also observed that a divalent Ca salt ionomer having an ion content of only 0.8 mol% induced shear deformation in addition to crazing. This is understood as arising from an increased effective strand density due to the formation of stronger ionic cross-links for the Ca salt ionomer as compared with the Na salt ionomer. Deformation mechanisms were also found to depend on thermal treatments of the ionomer specimens. When thin films were cast by a polar solvent, such as dimethylformamide (DMF), ionic aggregates were destroyed but a suitable annealing treatment permitted recovery of ionic aggregates, comparable to those formed in bulk ionomer specimens, and thereby altered the deformation mode.