Sodium sulfonated poly(styrene-ethylene- butylene) random ionomers [P(SEB-co-SSNa)] were obtained by hydrogenation, sulfonation, and neutralization of styrene-butadiene copolymers containing 45 wt % styrene. The dynamic mechanical properties were measured as a function of temperature for ionomers of a wide range of ion contents. Most of the ionomers showed two glass transitions, as well as a well-developed ionic plateau region in the storage modulus between the two glass transitions. The loss tangent peak of the upper T-g appeared below the decomposition temperature of the polymer, which permitted its detailed analysis as a function of ion concentration. The results of this study are explained on the basis of a recent model of morphology of ionomers in the solid state (the EHM model) and compared with those of the styrene-sodium methacrylate copolymers [P(S-co-MANa)] and sodium sulfonated polystyrene [P(S-co-SSNa)]. Considerable similarities exist in the properties of the matrix phase in the P(SEB-co-SSNa) and P(S-co-MANa) ionomer systems, in spite of the differences in composition. The P(SEB-co-SSNa) and P(S-co-SSNa) ionomer systems show similarities in the properties of the cluster phases, which, however, differ from those in the P(S-co-MANa). A detailed analysis of the storage modulus and the loss tangent curves showed that the storage modulus in the ionic plateau region is controlled by a filler effect of the dispersed phase (cluster or matrix) when there is only one continuous phase in the system. The storage modulus changes according to the logarithmic mixing rule when both the matrix and cluster phases are cocontinuous. Also, percolation theory can be applied to the modulus values of the P(SEB-co-SSNa) ionomer system.