In view of the separation and purification of salt streams, the permselectivity of ion-exchange membranes between different counter-ions is of interest. This paper investigates the permselectivity of two cation-exchange membranes (Neosepta CMS and Neosepta CMX). The separation between either sodium/calcium, sodium/protons or calcium/protons was studied and the influence of operating parameters was tested. It was found that a change in concentration ratio did not have a significant effect on the permselectivity. Furthermore, the monovalent selective membrane enables the preferential removal of sodium in an excess amount of calcium. The separation of sodium or calcium from protons is hampered because of the high mobility of protons in aqueous solutions. In the presence of weak bases, such as acetate and formate, the permselectivity (Na+ relative to H+) is increased more than 100 times. As a solvent either water or a solvent mixture of water with methanol or ethanol was used. Generally, the permselectivity changed according to the variation in ion mobility in the solvent mixture. The permselectivity between sodium and protons was enhanced, while the permselectivity between sodium and calcium was decreased. Hereby, the more nonpolar co-solvent, ethanol, has a larger influence than methanol. (C) 2016 Elsevier Ltd. All rights reserved.