This study involves modifying the surface of nanofiltration. (NF) membranes by ion implantation for increased salt rejection. F- ions were implanted on the surface of commercially available nanofiltration membranes to increase the negativity of the membrane surfaces. The objective was to increase the Dorman exclusion effect and thus increasing the salt rejection by the modified membranes. A composite polyamide NF membrane (NF 90 Filmtec) and a cellulose acetate NF membrane (SP 28 Osmonics) were implanted with F- ions at two different intensities-1E10 and 5E10 atoms/cm(2). Zeta potential measurements of the unmodified membranes and modified membranes show higher negativity with an increased intensity of ion implantation. Multicomponent salt permeation experiments were performed with the three types of membranes-the unmodified membrane and the two types of modified ones for both NF 90 and SP 28. The results show a decrease of solute flux for all the ions through the modified membranes when compared to that with the unmodified membrane. Initial experiments show no leaching of F- ions into the solution. Therefore, ion implantation on nanofiltration membrane surfaces is a novel technique to increase the salt rejection property of membranes. © 2005 Elsevier B.V. All rights reserved.