Amino acids can be transported selectively across a double membrane system composed of a cation-exchange and an anion-exchange membrane via pH adjustment of source phase solution. For amino acids, the isoelectric points mutually differ and their charges depend on solution pH. The diffusion flux of amino acids under a concentration gradient of hydrogen ion or hydroxide ion across an ion-exchange membrane is large but the source phase solution pH is altered by the counter-transport of hydrogen ion or hydroxideion. The solution pH alteration can be depressed using a double membrane system. The mutual separation of amino acids was conducted in the double membrane system, and the flux ratio of some amino acids was greater than 4000. Enrichment of amino acids was also conducted along with the consumption of hydrogen ion or hydroxide ion in the receiving phase without significant change of the source phase pH. (C) 2013 Elsevier B.V. All rights reserved.