The electrodialytic separation of a large molecular organic acid using highly water-swollen poly(vinyl alcohol) (PVA)/poly(styrene sulfonic acid-co-maleic acid) (PSSA-MA) cation-exchange membranes were investigated. The PVA/PSSA-MA membranes exhibited a lower membrane electrical resistance and a higher water-swelling property for large molecular organic ions (i.e., lysine-H+) compared to the commercial membrane, CMX (Tokuyama, Japan). While the ion-transport number (ITN) of the PVA/PSSA-MA membranes for Na+ was higher than 0.95, the ITN for lysine-H+ was measured as 0.96-0.98. Further, the energy efficiency in the electrodialysis for separating lysine was improved using the PVA/PSSA-MA membranes. The flux of lysine-H+ transported through the PVA/PSSA-MA membrane was measured as 19.8 mol m(-2) h(-1) while that of the CMX membrane was measured as 16.9 mol m(-2) h(-1) under the same power consumption (i.e. 4.0 x 10(4) W s). (C) 2003 Elsevier B.V. All rights reserved.