A new model is proposed to evaluate the separation performance of nanofiltration (NF) membranes for the mixed salts solution. In the model, the observed transmission of an ion through a NF membrane is applied to express the separation performance of the membrane for the ion in the mixed salts solution, which has a relationship with the total concentration, the equivalent fraction and the species of each ion in the mixed salts solution. The verification of the model was carried out in the permeation experiments of some mixed salts solutions ((I) Na+, Cl- and F-; (2) Na+, K+ and Cl-; (3) Na+, F-, Cl- and NO3- ; (4) Na+, Cl-, NO3- and SO42-) through three commercial NF membranes (ESNA 1-LF, ESNA1 and LES 90). According to the permeation experiments of three NF membranes for some binary salts solutions, the competition coefficients of ions were obtained. The model evaluation results agreed quite well with the experimental data. Finally, the model was applied to evaluate the observed transmission of each ion in the mixed salts solution (Na+, F-, Cl-, NO3- and SO42-) through three NF membranes. The agreement between the model evaluation results and the experimental data indicated that the model is suitable for evaluating the separation performance of three NF membranes for the mixed salts solution. (c) 2006 Elsevier B.V. All rights reserved.