Nanofiltration (NF) membranes operate with higher flux and lower energy requirements than reverse osmosis (RO) membranes, and exhibit relatively low rejection and selectivity for monovalent ions. In cases where high flux, high selectivity and high monovalent ion rejection are desired, an ideal membrane would exhibit the flux behavior of NF membranes and rejection behavior of RO membranes. Thus, a commercially available NF membrane was modified by layer-by-layer (LbL) assembly of alternating polyelectrolyte thin films in order to increase rejection and selectivity towards monovalent ions, namely F- and Cl-. Poly (styrene sulfonate) (PSS) was the anionic polyelectrolyte and poly (diallyldimethyl ammonium) chloride (PDADMAC) was the cationic polyelectrolyte. Thin (0.5-8.5) PDADMAC/PSS bilayers were deposited on the substrate membrane. NF experiments were performed in a cross-flow cell with feed solutions of (a) a ternary mixture of sulfate, fluoride and chloride at 100, 500 and 1000 mg/L. concentrations and (b) single salt solutions of Cl- or F-. In the ternary mixture experiment, fluoride rejection increased from 40% (unmodified) to 70% (8-bilayer modified), whereas chloride rejection was reduced from 55% (unmodified) to 20% (8-bilayer modified). The selectivity for chloride over fluoride was 2.7 for an 8-bilayer modified membrane compared to 1 for unmodified membrane. In single salt filtration of fluoride there was no appreciable change in the rejection for the modified membranes, whereas in the filtration of chloride the rejection increased from 30 to 91% upon modification (8-bilayer). The flux decreased by 30% with 8-bilayer modification for all feed solutions. The performance (flux, rejection, selectivity) of the modified membranes was compared to a BW30 RO membrane: under identical operating conditions, the modified membranes exhibit higher selectivity and flux than the commercial RO membrane. (C) 2011 Elsevier B.V. All rights reserved.