Lightly sulfonated polystyrene (7 mol %) (PS-SSA) that is ionically complexed with monodisperse tertiary amine-terminated oligomers of styrene, both mono- and bifunctional, were investigated by dynamic mechanical thermal analysis (DMA). The chain lengths of the oligomers are significantly greater than the average segmental length between the functional groups in PS-SSA. The monofunctional oligomer plasticizes PS-SSA as expected but considerably less than do alkyl chains, attributed to the greater bulkiness and stiffness of the styrene segments. The bifunctional oligomer, twice as long as the monofunctional oligomer, has very little plasticizing action and, at high concentrations, phase separates from the PS-SSA, despite extensive intercomponent interactions. This is related to a copolymer effect, suggested to be all the more pronounced because no cluster phase is apparent.