The aim of this work was to develop a simple and accurate model for predicting the concentration polarization index in the nanofiltration (NF)/reverse osmosis (RO) of dilute multi-ionic solutions. On the grounds of this model, the total flux of the ion i at the feed-solution/membrane interface consists of the sum of the diffusion, convection and migration fluxes, the former of which is determined by conventional mass-transfer correlations duly corrected to take into account the permeation through the membrane (suction effect). The coupling of the ionic fluxes is enforced by the electroneutrality requirement at the feed-solution/membrane interface. The model developed dispenses with the arbitrary assumption of the thickness of a film layer in the vicinity of the membrane surface. Assessing the accuracy/validity of this model with multi-ionic solutions would be rather harsh, thus the model accuracy and ranges of validity were ascertained for a benchmark case: NF/RO of single salt solutions. The model predicts approximate concentration polarization indexes of the salts A(+)B(-), A(2)(+)B(2-) and A(3)(+)B(3-) (or A(2)B(2)(-) and A(3+)B(3)(-)) with positive deviations lower than 10% with respect to the benchmark concentration polarization index, for ions diffusivities ratios, D-1/D-2 (or D-2/D-1), in the range 0.16-5.5 and phi J(v)/k(c) < 3, where J(v) is the permeation flux and k(c) is the mass-transfer coefficient of the salt for vanishing mass-transfer rates at impermeable walls. The main assumption of the model - the individual mass-transfer coefficients of the ions are independent of each other -appears to hold in a broad range of conditions, for single salt solutions. The model developed was expeditely applied to predict the concentration polarization in the nanofiltration of solutions containing Na+, Cl- and a dye(3-) (experimental data of Bowen and Mohammad [AIChE J. 44 (8) (1998) 1799-1812]), and its predictions are in fair agreement with the predictions of the extended Nernst-Planck equations in the film layer of the "slowest" ion. (C) 2007 Elsevier B.V. All rights reserved.