Abstract: In laboratory-scale experiments, rejection-flux curves of four different commercial membranes were established for four different model solutions (NaCl, CaCl2) and (NaCl + CaCl2) and for Ultrafiltration (UF) whey-permeate (pH = 4.6). The results indicated that the salt transport through all the NF membranes investigated depends on the flux. At low flux, when the contribution of diffusive transport is the most important, permeability of (specially monovalent) cations is high. At high flux, when transport by convection is the most important, rejection reaches a maximum (constant) value. From this it follows that the salt transport can be controlled by the flux. The parameters derived from the results with UF-whey permeate can be used to predict the salt rejection for similar multi-component systems like whey and UF-permeate in industrial systems.