Biphasic Cs and Ba carboxylated polystyrene ionomers were blended with chemically identical mono- and bifunctional oligomers whose chain lengths are significantly greater than the average segmental length between ionic groups in the ionomer. The ionomers were chosen to have two ion contents such that, in one case, the matrix phase is dominant and, in the other, the cluster phase is dominant. From dynamic mechanical thermal analysis, it was observed that the biphasic morphology of the ionomers is preserved in all blends (containing up to 60 wt % oligomer). The monofunctional oligomers tend to have a mild plasticizing action on the two phases. In the matrix-dominated ionomer, the bifunctional oligomer has an antiplasticization effect at lower oligomer contents; this is attributed to a greater dispersion of ionic aggregates in the matrix phase, accompanied by pinning of the oligomer extremities in multiplets, which prevents plasticizing action. In the cluster-dominated ionomer, the oligomer forms a separate phase, which is related to insufficiently large ion-poor styrene regions in the ionomer to accommodate the long nonionic portion of the oligomer chains.