Membrane nanofiltration is extremely complex and is dependent on the micro-hydrodynamics and interfacial events occurring at the membrane surface and within the membrane nanopores. There is significant debate as to the exact nature of these complex phenomena and rejection is typically attributed to a combination of steric and electrical effects. The electrical effects are less well understood and in particular the contribution of dielectric exclusion. There is a real need to determine a suitable description for dielectric exclusion for use in membrane modelling for the ab initio design and development of new membrane separation processes and for the scale up and optimisation of existing processes. In this paper the currently accepted separation mechanisms of membrane nanofiltration are explored in order to justify the use of porous models. Furthermore, the Desal-5-DK membrane is characterised by performing a series of rejection experiments of 0.01 M salt solutions at the membrane isoelectric point. This has the advantage of effectively neutralising the membrane fixed charge and facilitating an independent study of the dielectric partitioning at the membrane surface. A simplistic model based on Born theory was tested and found to be remarkably accurate, indicating that ion solvation may be the appropriate dielectric exclusion mechanism for true nanofiltration membranes. (C) 2012 Elsevier B.V. All rights reserved.