From extraction experiments and γ-activity measurements, the exchange extraction constants corresponding to the general equilibrium M+(aq)+NaL+(nb)⇄ML+(nb)+Na+(aq) taking place in the two-phase water-nitrobenzene system (M+=Li+, K+, Rb+, Cs+; L=18-crown-6; aq = aqueous phase, nb = nitrobenzene phase) were evaluated. The stability constants of the ML+ complexes in nitrobenzene saturated with water were calculated; they are found to increase in the cation order Cs+Li+Na+Rb+K+. Further, the individual extraction constants for the NaL+, KL+, RbL+ and CsL+ complex species in the wate-nitrobenzene system were determined; their values increase in the series Na+Rb+Cs+K+.