Equilibria of ion exchange of neutral aliphatic amino acid cations (A(+) = Gly(+), Ala(+), Val(+), Leu(+)) and H+ have been studied on a liquid sulfonic type ion exchanger -dinonylnaphthalene sulphonic acid dissolved in heptane. It has been established that the difference in selectivity of extraction of these cations is much greater than on sulphostyrene resins, which is a favourable precondition for their selective extraction. The selectivity coefficients of the H+-A(+) exchange have a region of a low dependence on the degree of loading of liquid ion exchanger ((X) over bar = 0-0.5) and a region of a sharp decrease ((X) over bar > 0.5). This phenomenon is explained as a result of formation of very stable complexes of the amino acid cations and the anion of dinonylnaphthalene sulphonic acid containing one unsubstituted proton per 3-6 A(+) cations. The LR spectra confirm this assumption. The dependence of the selectivity coefficient on the degree of H+-A(+) exchange has been described using the previously developed mathematical model of ion-exchange processes. The model is consistent with the experimental data in an assumption that four nearest neighbours of an exchange centre in the associates of the liquid ion exchanger influence the energy of ion exchange on a given exchange site.