In this study, the retention of nitrate ions by a commercial nanofiltration membrane is investigated to highlight their transport mechanism. The influence of the operating pressure and the ionic composition (nature of the associated ion, feed concentration, pH and solution complexity) are studied. The separation performance should be the result of dual forces: a global surface force (friction, electrostatic) which retains the solute in the pore and a global convective force directed towards the retentate which depends on the applied pressure. In binary solutions, surface force depends on the hydration energy of the associated cation as well as membrane-solute electrostatic interactions. In ternary mixtures, we underscored the influence of solute-solute interactions (Donnan effect). The observations made with simple nitrated solutions (binary and ternary) were extrapolated and confirmed in experiments made on polluted or nitrates doped waters. (C) 2003 Elsevier B.V. All rights reserved.