In contrast to other anions, there are not many studies focusing on the effect of bicarbonate ion on the rejection of other anions in nanofiltration. However, bicarbonate is one of the major anions in many inland waters and its concentration remains significant even after acidification. In order to obtain specific information about the interaction of bicarbonate and nitrate ions, nanofiltration experiments with the membrane SU-610 (Toray) were performed using solutions containing sodium, nitrate and bicarbonate ions. The influence of the bicarbonate ion concentration was studied in the range from 4 to 32 mol. m(-3) of bicarbonate ion and from I to 4 mol.m(-3) of nitrate ion. The experimental data were analysed by means of multiple linear regression. The results showed that nitrate rejection was decreased by bicarbonate ion concentration. This effect was compared with that obtained using sulphate or chloride as co-ions. The regression models obtained for nitrate and bicarbonate ion rejection showed good fitting to the experimental results (r-squared equal to 99.3% and 97.6% respectively). The models were able to predict the evolution of nitrate in a concentration process at constant pH. (C) 2009 Elsevier B.V. All rights reserved.