T he feasibility of the separation of peptides by ion exchange chromatography using an aqueous solution of carbon dioxide in water as displacer has been studied. Ion exchange equilibria have been measured for amino acids and dipeptides. Elutions at increasing pressures show the increase of recovered product concentration. Feasibility of displacement separations and dependence upon a global separation factor which takes into account both equilibria in the solution and affinities of the support towards the various molecules are proved. This factor is thus only based on thermodynamic considerations. In case it proves the feasibility of the separation, kinetic limitation considerations had to be taken into account to achieve the best separation.