The sorption equilibria of glutamic acid (Glu). methionine (Met), and lysine (Lys) on a cation-exchange membrane, SELEMION-CMV, and an anion-exchange membrane, SELEMION-AMV, have been investigated over a wide range of pH. The cation-exchange membrane showed increasing selectivity from solutions containing HCl in the order Glu, Lys, and Met, but showed only very slight sorption of Met and Glu from NaCl solutions. With anion-exchange membrane, the sorption from NaCl solution increased in the order Lys, Met, and Glu as pH increased from 8 to 11. The dependence of amino acid ionic form on pH provides a qualitative explanation of those results. A quantitative selectivity coefficient model was then determined, based on the solution and ion-exchange equilibria for each amino acid, and was found to simulate experimental results quite successfully, thereby proving that amino acids are stoichiometrically sorbed by ion-exchange membranes.