Electrodialysis experiments were carried out to achieve efficient production of lactic acid from sodium lactate. Conventional electrodialysis (CED) consisting of cation- and anion-exchange membrane and ion substitution electrodialysis (ISED) consisting of only cation-exchange membrane were performed and the results were compared in terms of lactic acid loss and the ratio (eta(1)) of sodium ions transported to the current supplied. While both electrodialysis operations removed over 95 % of sodium ions from the feed solution, the CED operation. however, resulted in a considerable loss of lactic acid. whereas there was no such loss in the ISED operation. Furthermore it was found that the ratio of sodium ions transported to the current Supplied was above 1.5 during the early stage of operation in the ISED experiment, indicating that the sodium ions are transported not only by electric force but also by concentration gradient in the ISED operation. By comparing the results of the two electrodialysis operation modes, it was found that the ISED operation is advantageous for the production of lactic acid from sodium lactate in terms of preventing lactic acid loss. In the ISED operation, however, process efficiency decreased with the accumulation of sodium ions in the acid compartment because sodium ions in the acid solution are transported back to the feed stream. This finding suggests the desirability of using a proton permselective cation-exchange membrane, which is preferentially permeable to proton ions among the various cations present in the solution. (C) 2002 Elsevier Science B.V. All rights reserved.