The influence of ion content and ionomer morphology on deformation modes of poly(styrene-co-sodium methacrylate) ionomers has been examined. Dynamic mechanical thermal analysis indicates a morphology characteristic to amorphous ionomers : two separate loss peaks are observed, one associated with the glass transition of the multiplet-containing matrix phase and the higher temperature loss peak with the glass transition of an ion-rich cluster ’phase’. As ion content rises, the intensity of the matrix loss peak diminishes, while the intensity of the cluster loss peak rises. At a critical ion content of about 6 mol%, the cluster phase begins to dominate. Such ’two-phase’ morphology of the ionomers has an appreciable effect on deformation modes. At low ion contents, where the matrix phase is the major one, only crazing is observed and the craze morphology is generally similar to that of polystyrene. At higher ion contents where the cluster phase becomes dominant, some shear deformation is present and the crazes tend to be shorter and less well defined. The effect of thermal treatment on crazing behaviour of the ionomers is also studied; two competing factors, i.e. simple physical aging and enhancement of the ion-rich cluster phase, determine the overall deformation modes. In general, thermal treatment enhances cluster formation, which in turn suppresses crazing and shifts the deformation mode towards shear deformation. Results of this study are compared with those reported on other types of ionomers, such as partially sulfonated polystyrene ionomers and ionomers based on poly(methyl methacrylate).