This paper analyses the separation properties of various commercial cation exchange membranes (CEMs) and tailor made membranes based on sulphonated poly(ether ether ketone) and poly(ether sulphone) for binary electrolyte solutions containing protons and calcium ions. All membranes are thoroughly characterised and relations between their electrochemical properties and ion selectivity are drawn. The effect of current density and calcium ion concentration in the feed stream on the membrane selectivity is investigated. Our results show that the conductivity and the charge density of the membranes determine the calcium transport through the membranes. The calcium transport increases with increasing conductivity, however, it is lower for membranes with lower charge density. Therefore, the preparation of membranes with reasonable conductivities and low calcium transport is possible. Besides, the calcium flux increases with the increase of current density and/or calcium concentration for all membranes. For CEMs having a positively charged coating, the calcium flux is low at low current density but increases strongly at high current densities. (c) 2005 Elsevier B.V. All rights reserved.