This study explored the influence of feed pH on permeate pH and rejections of ions under acidic conditions in NF process. A Desal-5 NF membrane was tested. Town water, 10 mM NaCl and 10 mM NaNO3 in deionized water were used as raw feed. Feed pH in the range of 1-7 was investigated. A critical feed pH was observed for each solution. Above the critical pH value NF permeate pH was lower than feed pH, but below the critical pH value permeate pH was higher than feed pH. The critical pH value for town water, NaCl and NaN03 solutions were 4.25, 4.50 and 4.55, respectively. These are close to the isoelectric point (4.0) of the NF membrane used. An explanation of these observed results, that have not been reported so far, is offered. The experimental results showed that rejection of NaCl in town water was lower than that in pure NaCl solution due to higher rejection of bivalent ions in town water. For NaNO3 solution at a feed pH below 1.6, a lower pH of permeate than feed was observed again due to positively charged groups on the membrane being shielded by anions at high salt concentration and significant reduction of the electrostatic repulsion between protons and the positively charged membrane surface. Ion rejection first passed through a minimum and then a maximum with a decrease in feed pH. Moreover, operating pressure and feed velocity in NF process did not appear to influence the relationship between feed pH and permeate pH. In addition, it was observed that rejection of NaCl in town water was lower than in pure NaCl solution due to the influence of co-existing divalent ions on reducing the rejections of monovalent ions. It was concluded that feed pH significantly influenced the electrostatic repulsion between the charged membrane surface and ions and therefore, permeate pH and ion rejection in NF process. A maximum in ion transmission corresponded to a peak in membrane flux and vice versa. Change in osmotic pressure does not satisfactorily explain the change in flux with pH. (C) 2004 Elsevier B.V. All rights reserved.