A semi-empirical model was proposed to predict the separation performance of NF membranes for mixed electrolytes solutions. The competition coefficient, regulation coefficient and dissociation proportion were discussed respectively for mixed electrolytes solutions with univalent cations, multivalent cations and weak electrolytes. To obtain and confirm these coefficients, permeation experiments of three binary electrolytes solutions (1, Na+, F- and Cl-; 2, Na+, Mg2+ and Cl-; 3, Na+, Cl- and HCO3(-)) were carried out through two commercial NF membranes (ESNA1-LF and LES 90). Finally, this model was verified in permeation experiments of four complex mixed electrolytes solutions (1, Na+, F-, Cl-, NO3(-), and SO42(-); 2, Na+, K+, Mg2+, Ca2+, and Cl-; 3, NH4+ (or NH3 center dot H2O), Na+, Mg2+, and Cl-; 4, Na+, K+, Mg2+, Ca2+, Cl-, NO3-, SO42-, and HCO3- (or CO2 center dot H2O)) through these two NF membranes. The agreement between model prediction results and the experimental data indicated that this model was suitable for predicting the separation performance of these two NF membranes in mixed electrolytes solution considered. (C) 2010 Published by Elsevier B.V.