On the basis of extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium K + (aq)+1·Na + (nb) ⇄ 1·K + (nb)+Na + (aq) taking part in the two-phase water–nitrobenzene system (1=antamanide; aq=aqueous phase, nb=nitrobenzene phase) was evaluated as log K ex (K + ,1·Na + )=0.2±0.1. Further, the stability constant of the 1·K + complex in nitrobenzene saturated with water was calculated for a temperature of 25°C: log β nb (1·K + )=4.7±0.2. Finally, by using quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1·K + was derived. In the resulting complex, the “central” cation K + is bound by four bond interactions to the corresponding four carbonyl oxygen atoms of the parent ligand 1. Besides, the whole 1·K + complex structure is stabilized by two intramolecular hydrogen bonds. The interaction energy of the considered 1·K + complex was found to be −346.6kJ/mol, confirming also the formation of this cationic species.