The small-angle X-ray scattering profile of styrene-isoprene-sulfonated isoprene terpolymer ionomers was studied to clarify the structure of ionic aggregates at ambient temperature as a function of the degree of neutralization of Na or Mg cations. An ionic cluster peak was observed for ionomers with a degree of neutralization greater than 25%. The ionic cluster peak was analyzed by the modified hard sphere model proposed by Yarusso and Cooper [Macromolecules, 1983, 16, 1871], and the size of the ion cluster, the closest approach distance between the clusters, and the average system volume per ion cluster were evaluated by a curve-fitting method. The size of the ion clusters of the ionomer with monovalent Na cation increased with the degree of neutralization, but for divalent Mg cation slightly changed. The number of ion clusters of the styrene-isoprene-sulfonated isoprene ionomer with Na and Mg cations characteristically increased with the neutralization.