The interaction of monovalent cations and membrane phospholipids has a great theoretical and practical importance. Because of the very low differences between the binding forces of monovalent cations to phospholipids, the methods applied up till now did not succeed in completely elucidating this interaction. We developed a new thin-layer chromatographic method to study the interaction of monovalent cations and phospholipids. Silica gel layers were impregnated with a solution of the phospholipid to be investigated (phosphatidylserine or phosphatidylcholine) to produce a multibilayer surface with the polar groups on the outside. 22Na+, 42K+, S6Rb÷, 137Cs + salts were then applied at the origin and chromatographed in a solvent system presaturated with the phospholipid. Our results indicate that phosphatidylcholine and phosphatidylserine bind the monovalent ions with smaller hydrated ion radii more strongly. The binding forces decrease in the order Cs + > Rb + > K + > Na + for both phospholipids. The binding forces are greater for phosphatidylserine than for phosphatidylcholine. The differences between the binding forces of monovalent cations to phosphatidylcholine are greater than to phosphatidylserine indicating that phosphatidylcholine differentiates relatively better between the monovalent cations than phosphatidylserine.