This paper discusses the performance of NF270, NF90, BW30 and SW30 (Dow/FilmTec) membranes during filtration of single and mixed NaCl and Na2SO4 solutions at different feed concentrations and NaCl to Na2SO4 ratios. The feeds represent the compositions of possible streams after IX treatment in a combined ion exchange (IX)-nanofiltration (NF) process of water desalination. The impact of total feed concentration, the NaCl/Na2SO4 ratio and feed pH on membranes flux and rejection of sulphate and chloride ions were determined. All membranes showed excellent rejection of sulphate ions, while chloride rejection largely varied for NF membranes with changes in feed concentration, composition and pH. A high negative rejection of chloride ions was observed with NF270 membrane during treatment of binary aqueous solution with a high Na2SO4 to NaCl ratio due to the Donnan effect. (C) 2014 Elsevier B.V. All rights reserved.