From extraction experiments and c-activity measurements, the exchange extraction constants corresponding to the general equilibrium M (aq) ? NaL (nb) , ML (nb) ? Na (aq) taking place in the twophase water–nitrobenzene system (M = H3O , NH4 þ, Ag, Tl; L = hexaethyl p-tert-butylcalix[6]arene hexaacetate; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. Furthermore, the stability constants of the ML complexes in nitrobenzene saturated with water were calculated; they were found to increase in the following order: Agþ \ NH4 \ H3O \ Naþ \ Tlþ: