The transport properties of nanofiltration (NF) membranes are investigated by means of an improved transport model including dielectric exclusion in terms of both Born dielectric effect and image force contribution. The SEDE model (Steric, Electric and Dielectric Exclusion model) can be used to describe transport through pores of cylindrical or slit geometry. The coupling between the various mechanisms involved in the retention phenomenon is pointed out and discussed. The volume charge density of a NF polyamide membrane is determined from tangential streaming potential measurements. The SEDE model is then used to assess the rejection rate of the membrane, the dielectric constant of the solution filling pores being the single adjustable parameter of the model. It is clearly shown that the dielectric exclusion cannot be neglected in the analysis of the filtration properties of NF membranes. Indeed, the classical steric/electric exclusion theory is found to be unable to describe experimental rejection rates of the polyamide membrane under consideration. (c) 2004 Elsevier B.V. All rights reserved.