Ion-exchange equilibria involving Mg2+/H+, Ca2+/H+, Sr2+/H+ and Ba2+/H+ in aqueous solutions have been studied on Nafion-117 and Dowex 50W X8 by batch technique at 298 +/- 1 K. Thermodynamic equilibrium constants have been calculated taking into account the specific ionic interactions in solution phase, which affect the corresponding ionic activities. These data are compared with similar data available in literature. The following selectivity sequence for alkaline earth metal ions have been observed: Ba2+ > Sr2+ > Ca2+ > Mg2+. The overall equilibrium constants for Mg2+/H+ and Ca2+/H+ exchanges on Nafion-117 are very small as compared to those on other ion exchangers having -SO3- as a functional group. The results are discussed in terms of the entropy changes taking place during ion exchange and formation of ionic aggregates in ionomers. It is likely that ion aggregation is responsible for the high values of equilibrium constants obtained for Sr2+/H+ and Ba2+/H+ exchanges on Nafion-117 in the present study.