The molecular weight dependence of the viscoelastic properties of polystyrene-based ionomers was examined. To measure the physical properties, a Rheometrics dynamic analyzer (RDA) was utilized for the poly(styrene-co-sodium methacrylate) (4 mel. %) system, while for the poly(styrene-co-sodium styrenesulfonate) samples (5 mol %) the studies were performed on a dynamic mechanical thermal analyzer (DMTA). :In the methacrylate samples of high molecular weight (>100 000) the plot of tan delta vs temperature showed two peaks, as in other ionomers. However, for samples of the lowest molecular weight (ca. 21 000), the tan delta plot showed only one peak and a shoulder. Peak deconvolution, however, showed that, even in that sample, there are two peaks and that the deconvoluted peak areas do not change significantly with molecular weight. The flow behavior of the matrix in the low molecular weight samples partially masks the relaxation process of the clusters. In the terminal region, the viscosities of the ionomers are molecular weight dependent in a way similar to that in nonionic polystyrene; above the critical molecular weight for entanglements, compliance values of the ionomers are independent of the molecular weight. :In the Case of the sulfonates, many trends are similar to those seen in the methacrylates. As the molecular weight increases, the cluster tan delta peak becomes more visible, and slopes,of storage modulus:curves in the terminal region become less steep. However, there are some differences, which are due to the difference in the strength of the interaction between ionic groups in the multiplets. In the methacrylates, because of weaker interactions, the ionomers start to exhibit appreciable ion-hopping and chain mobility at a relatively low temperature (Ca. 170 degrees C), which is the onset of the glass transition (T-g) of the cluster phase and is only ca. 50 degrees C higher than the matrix T-g. Above that temperature, the multiplets,can no longer act as long-term cross-links, and the ionomers show a strong molecular weight dependent behavior. On the other hand; in the sulfonates, due to the stronger interaction, ion-hopping becomes important only at much higher temperature (ca : 220 degrees C), which is, again, the beginning of the cluster T-g; above that point these ionomers also start to show appreciable molecular weight dependent behavior. In addition, in the sulfonates, the activation energies:for the matrix and cluster transitions and for ion-hopping are independent of the molecular weight. All these-results suggest that the stability of multiplets is independent of the molecular weight. The thermal stabilities of the present ionomers are similar to that of polystyrene.