From extraction experiments and c-activity measurements, the extraction constant corresponding to the equilibrium Cs ? (aq) ? 1� Na 1 (nb) = 1� Cs ? (nb) ? Na ? (aq) taking place in the two-phase water-nitrobenzene system (1 = lithium ionophore VIII; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (Cs ? , 1� Na ? ) =- 0.5 ± 0.1. Further, the stability constant of the 1� Cs ? complex in nitrobenzene saturated with water was calculated for a temperature of 25 C: log bnb (1� Cs ? ) = 4.8 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1� Cs ? was derived. In the resulting complex, the ''central'' cation Cs ? is bound by six bond interactions to the corresponding six oxygen atoms of the parent ligand 1.