From extraction experiments and γ-activity measurements, the extraction constant corresponding to the equilibrium NH 4 + (aq)+1·Na + (nb)⇆1·NH 4 + (nb)+Na + (aq) taking place in the two-phase water–nitrobenzene system (1=enniatin B; aq=aqueous phase, nb=nitrobenzene phase) was evaluated as log K ex (NH 4 + , 1·Na + )=1.9±0.1. Further, the stability constant of the 1·NH 4 + complex in nitrobenzene saturated with water was calculated for a temperature of 25°C: log β nb (1·NH 4 + )=6.4±0.2. Finally, applying quantum mechanical DFT calculations, the most probable structure of the cationic complex species 1·NH 4 + was derived. In the resulting 1·NH 4 + complex, the “central” cation NH 4 + is bound by three relatively strong hydrogen bonds to the corresponding three carbonyl oxygens of the parent enniatin B ligand. The interaction energy, E(int), of the considered complex 1·NH 4 + was found to be −305.5kJ/mol, confirming also the formation of this investigated complex.