A new concept for the synthesis of thin polyamide nanofiltration membranes on ultrafiltration polysulfone supports is reported. Polyamide membranes with controllable thin dense layer and effective "nanopores" were fabricated by adding co-solvent (acetone) to nonpolar organic phase (hexane), referred to as co-solvent assisted interfacial polymerization (CAIP), which exhibited a high water flux with no considerable salt rejection loss Higher co-solvent addition into the hexane solution resulted in relatively larger pore sizes as well as higher water fluxes, which were determined by the analysis of membrane permeation data using aqueous solutions of sodium chloride. The best nanocomposite membrane that was prepared with 2 wt% of acetone showed approximate 4 times higher water flux with no considerable rejection loss than polyamide membranes fabricated with no acetone co-solvent The CAIP method will offer new degrees of freedom in modifying polymer membrane with high separation performance (C) 2010 Elsevier B V All rights reserved