The dynamic mechanical properties of poly(styrene-co-sodium methacrylate) ionomers were reexamined in detail and the data interpreted in terms of filler and percolation concepts using the Eisenberg-Hird-Moore (EHM) multiplet/cluster model of random ionomers. For this study, samples were synthesized over a wider range of ion concentrations as well as more frequent intervals than in the previous studies. Deconvolutions were performed on the loss tangent data. The glassy moduli of the ionomers were found to be independent of the ion content. There were discontinuities in the plots of the slopes of the storage moduli, peak heights, peak positions, and widths at the half-height of the loss tangent peaks as well as of the activation energies for the glass transitions as a function of ion content. These discontinuities suggest that two morphological changes are involved in the present ionomer system. The first, at 4-6 mol % of ions, involves the formation of a dominant or a continuous phase of the ion-rich or cluster regions and is interpreted as being associated with the percolation threshold (see below). The second, at approximately 12-14 mol % of ions, possibly involves the disappearance of continuity of the unclustered "phase"; between 4 and 12 mol %, two cocontinuous "phases’ are probably encountered. Differential scanning calorimetry (DSC) thermograms for the ionomers between 8 and 14 mol % of ions show two glass transition temperatures (T(g)). A linear relation is observed between these two T(g)s and the positions of the E" peaks at 0.3 Hz obtained from dynamic mechanical thermal analysis (DMTA) measurements. In terms of filler concepts, the Guth equation is applicable in the range of low volume fractions of clusters (<0.3), but a minor modification extends the applicability to 0.45. Application of the Halpin-Tsai equation for regular systems to the present ionomer at low ion content suggests that the system consists of more or less spherical (at opposed to linear or lamellar) clusters dispersed in the matrix phase as filler particles. The mechanical properties of ionomers were also interpreted in terms of percolation concepts. At the percolation threshold, i.e., at approximately 5 mol % of ions, the critical exponent and the critical volume fraction of clusters were found to be 1.31 and 0.64, respectively. The value of the critical exponent is in the range of "universal" values for conductivity percolation, but the critical volume fraction is much higher than those for most other systems. This is explained in terms of the difference between the nonuniformity of properties of the clusters of the present system and the uniformity of properties in usual percolating species. Finally, a novel approach is used to estimate the size of a sodium carboxylate ion pair, which is found to be 45 X 10(-3) nm3.