Two tight nanofiltration membranes of 100 and 400 Da were tested at different operating conditions. The results show that the selective properties of nanofiltration membranes come from a combination of several physicochemical mechanisms. Mass transport is mainly diffusive in the 100 Da membrane, while two mechanisms of segregation exist in the 400 Da membrane. Large molecules are retained at the membrane surface by electrostatic and steric interactions and small, weakly charged ions can enter the pores. In these pores, the segregation is due to surface forces (electrostatic and friction) and the solvent can transfer by a convective mechanism. (C) 1997 Elsevier Science B.V.