Separation of amino acids by Donnan dialysis using an ion-exchange membrane was studied. Donnan dialytic experiments were carried out using a commercial anion-exchange membrane Neosepta AM(-1) (in OH-form), glutamic acid-alanine mixed solutions as the feeds, and sodium hydroxide solutions as the stripping ones. The amino acid fluxes were then measured. The initial concentrations of the two kinds of amino acids in the feed were equal, while the sodium hydroxide concentrations of the stripping solutions were widely varied. The glutamic acid flux was greater than that of alanine under all experimental conditions. The glutamic acid flux increased with the stripping solution concentration and was roughly constant in the region above a certain concentration. On the other hand, the alanine flux monotonically increased with the concentration of NaOH. The flux ratios of glutamic acid to alanine also varied with the concentration, but had maximum values. It was found that not too high concentration of NaOH was essential to separate the amino acids in the Donnan dialysis. An upper limit of the appropriate concentration of stripping solution, in which the flux ratio was greater than 100, became higher with the amino acid concentration in the feed. Furthermore, the reason why the flux ratio decreased with the increasing stripping solution concentration was discussed. (C) 2007 Elsevier B.V. All rights reserved.