The effects of the presence of dimer acid (DA) molecules in Na-sulfonated polystyrene (PSSNa) and poly(styrene-co Na-methacrylate) (PSMANa) ionomers on the ionomer properties were investigated using a dynamic mechanical analysis technique. The DA decreased the matrix and cluster Tgs of PSSNa ionomer without changing the ionic modulus, suggesting that the DA resided both in the matrix and cluster regions of PSSNa ionomer and acted mainly as a plasticizer. On the other hand, the DA decreased the cluster Tg of PSMANa ionomer, without changing the matrix Tg, and also increased the ionic modulus, implying that the DA behaved like plasticizer only for the cluster regions. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013