From extraction experiments and c-activity measurements, the exchange extraction constants corre- sponding to the general equilibrium M 2þ aq ðÞ þSr 2þ org ðÞ , M 2þ org ðÞ þSr 2þ aq ðÞ taking place in the two-phase water-phenyltrifluoromethyl sulfone (abbrev. FS 13) system (M 2? = Mg 2? ,C a 2? ,B a 2? ,C u 2? ,Z n 2? ,C d 2? ,P b 2? , UO 2þ 2 ,M n 2? ,F e 2? ,C o 2? ,N i 2? ;a q= aqueous phase, org = FS 13 phase) were evaluated. Furthermore, the indi- vidual extraction constants of the M 2? cations in this two- phase system were calculated; they were found to increase in the series of Mg 2? ,UO 2þ 2 \ Ca