Results of dynamic mechanical studies and measurements of physical properties of the ionic polymer based on carboxylated nitrile rubber (XNBR) and zinc oxide (ZnO) reveal that dioctylphthalate (DOP) plasticizes the backbone chain of the polymer, while dimethylsulfoxide (DMSO) plasticizes the hard phase arising from the ionic aggregates. Accordingly, the high-temperature ionic transition in the plot of tan delta versus temperature disappears at high DMSO loading, while the glass-rubber transition temperature (T-g) is little affected. Incorporation of DOP, on the other hand, causes shifting of T-g to lower temperature, without affecting the high-temperature transition. Both plasticizers cause a drop in physical properties, but the drop is greater in the case of the DMSO-plasticized system.