The effect of gel layer thickness on salt separation of positively charged pore-filled nanofiltration membranes has been examined both theoretically and experimentally. The extended Nernst-Planck (ENP) equation coupled with the Teorell-Meyer-Sievers (TMS) model were used to calculate the pressure-driven sodium chloride rejections for membranes having gel densities in the range typically used in nanofiltration applications. It was found that salt rejection was dependent on membrane (gel-layer) thickness with salt rejections increasing rapidly with thickness up to 50-75 mu m. Further increases in thickness beyond this point had a much smaller effect on salt rejection. The theoretical predictions were examined experimentally by preparing a series of membranes with cross-linked poly (3-acrylamidopropyl)-trimethylammonium chloride (PAPTAC) gels with varying densities within the pores of a thin microporous polyethylene (PE) support. The membranes were characterized by their polymer volume fractions (gel concentration), thicknesses and effective charge densities. The effect of membrane thickness was examined by using single and stacks of two membranes. The pure water fluxes and salt rejections of the membranes and membrane stacks were determined in the pressure range 50-550 kPa. The single salt rejections of the membranes which were very dependent on the thickness of the membrane or membrane stack, were fully in accord with the calculated salt rejections of the membranes. (c) 2007 Elsevier B.V All rights reserved.