The aim of the present paper is the comparison of electrodialysis (ED) and ion exchange (IE) with regard to their cumulative energy demand by means of two applications separating ions with significantly different molar weight. Data for the separation of sodium gluconate and for the desalination of whey are obtained from laboratory experiments. Therefrom, a scale-up is done to an industrial-scale plant, which has been verified for the desalination of whey by comparison with data from a commercial plant. The obtained cumulative energy demands consist of the specific energy demand for the transport of ions across the membranes, for pumping, and for the waste-water treatment in the case of ED, and of the specific energy demand for pumping, for the production of regenerants, and for the waste-water treatment in the case of IE. Applying ED, cumulative energy demands of 97 and 91.7 Wh/mol result for the separation of sodium gluconate and for the desalination of whey, respectively. With IE, cumulative energy demands of 291.1 and 190.5 Wh/mol result for the separation of sodium gluconate and for the desalination of whey, respectively. Thus, ED is significantly better with regard to sustainability, expressed in their cumulative energy demand, if ions with a molar weight between approximately 20 and 200 g mol(-1) have to be separated. Generally, an increase of the two cumulative energy demands occurred separating sodium gluconate. (C) 2003 Elsevier B.V. All rights reserved.