Zinc sulfonated ethylene-propylene-diene terpolymer (EPDM) behaves as an ionic thermoplastic elastomer. Both paraffin oil and dimethyl sulfoxide (DMSO) act as plasticizers for this polymer; while paraffin oil plasticizes the hydrocarbon backbone, DMSO plasticizes the ionic domains. Results obtained from dynamic mechanical analysis (d.m.a.) show that DMSO does not affect the glass-rubber transition temperature (T-g), occurring at around -26 degrees C, but adversely affects the broad transition in the temperature range from +27 to +80 degrees C, which is believed to be due to the hard phase resulting from the ionic aggregates in the polymer. Paraffin oil, however, causes a lowering of the T-g (from -26 to -34 degrees C) without affecting the high-temperature transition. Results obtained from dielectric thermal analysis (d.e.t.a.) indicate the existence of two types of ionic aggregates, namely multiplets and clusters which are responsible for the occurrence of two types of dipolar relaxation in the ionic-transition zone. Processability studies using the Monsanto processability tester (MPT) show that DMSO causes a higher extent of plasticization than paraffin oil. Incorporation of DMSO also leads to greater loss in physical properties of the zinc sulfonated EPDM when compared to paraffin oil.