Two new nanofiltration membranes, an organomineral (titania-Nafion) medium and an inorganic (gamma Al2O3) medium, were studied using single or mixed solutions of monovalent or divalent ions. The experimental results not only confirm previous findings concerning the treatment of electrolytic solutions using charged nanofiltration membranes, but provide additional data notably for microporous membranes. A simple model based on the Nernst-Planck equation is proposed; ifs simplicity is related to the nature of the two membranes tested for which the diffusion terms may easily be disregarded. It highlights the importance of the potential difference that forms on the membrane and explains most of the observed rejection results. In addition, ionic plugging of micropores may play a part with regards to rejection and flow depending on working conditions.