The rejection of single electrolytes at six nanofiltration membranes has been experimentally studied. The membranes were chosen to cover the range from ultrafiltration to reverse osmosis and to represent a diversity of polymers used for membrane fabrication. Such experimental data has been interpreted using a model based on the extended Nemst-Planck equation. The model accounts for the hindered nature of transport in the membranes. Such an interpretation allows a characterisation of the membranes in terms of two parameters, an effective membrane thickness and an effective membrane charge density. The latter may be related to the solution ionic content by means of an isotherm. The method presented also allows an estimation of the effective pore size of the membrane. A knowledge of the effective membrane thickness, effective charge density and effective pore size allows use of the model to predict the separation of mixtures of electrolytes at a membrane. Very good agreement between such a prediction and experimental data has been obtained.