In this paper, the transport mechanism of monovalent anions, fluorides, chlorides and nitrates particularly, through three commercial nanofiltration membranes is studied. By increasing the complexity of the solution to be treated, we underline the influence of hydration and electrostatic interactions on ions retention. Depending on membrane and on its charge density, these influences will be more or less pronounced. In brackish water conditions, the shielding of membrane charge is higher than in fresh water, which lies to a weaker Donnan exclusion and a greater influence of size effect. (C) 2004 Elsevier B.V. All rights reserved.