Zeta potential measurements were used to calculate the surface charge of nanofiltration membranes. This surface charge was incorporated in a model based on the Teorell-Meyer-Sievers theory to calculate the rejection of nanofiltration membranes under the assumption of constant membrane potential. The model was fitted using NaCl rejection measurements at pH 7 for a salt concentration of 5 mmol/l to determine two membrane parameters, the pore radius and the ratio of pore density and membrane thickness. The membranes tested were the Desal 5DK and the PVD1 nanofiltration membranes with a molecular weight cutoff below 300. The membranes are positively charged below the isoelectric point around pH 4 and negatively charged at higher pH values. Using the fitted parameter, the calculated rejection is compared with experiments for different concentrations at pH 7, for ternary ion mixtures and for different pH values. The concentration dependant rejection for NaCl could be well predicted in the concentration range between 2 and 50 mmol/l. The agreement was also very good for calculated and measured rejections of NaCl and Na2SO4 mixtures. A minimum in rejection was found for both membranes, which corresponds well with the isoelectric point in the case of the PVD1 membrane, but which is at a higher pH for the Desal 5DK membrane. The NaCl rejection as a function of pH could be well predicted for the PVD1 membrane, but there are considerable deviations in the low pH range for the Na,SO, rejection for the same membrane as well as for the NaCl rejection for the Desal5 DK. The method presented also gives some insight in the mechanisms leading to the abrupt changes in rejection around the isoelectric point of the membranes.