Polyethersulfone ultrafiltration membranes were converted into charged nanofiltration membranes having a strong polyelectrolyte hydrogel as selective barrier layer through the UV-photo initiated graft polymerization technique. This was accomplished by using vinyl sulfonic acid as the functional monomer and N,N'-methylenbisacrylamide as a cross linker monomer (Bernstein et al., ACS Applied Materials 82 Interfaces, 4 (2012) 3438-3446). In this research the resulting composite membranes were further characterized using different methods (ATR-FTIR spectroscopy, zeta potential, contact angle, scanning electron microscopy). ATR-FTIR data were used to quantify the degree of grafting. The composite membranes' zeta potential was negative throughout the pH range and as high as -70 mV. The hydrogel composite membranes were also very hydrophilic with a contact angle of 11 degrees. The membrane performance-salt rejection and water permeability-obtained at varied functionalization conditions-molecular weight cut-off of the base membrane, monomer concentration, cross linker fraction, UV irradiation intensity and time-was systematically investigated and the results were correlated to the membrane characterization data. Separation performance was also tested using mixed salt solutions. Larger composite membrane samples were prepared and long-term stability of nanofiltration (NF) performance was evaluated in cross-flow experiments. The performance of the best of the newly fabricated composite membranes was comparable to other polyelectrolyte-based NF membranes as well as to some commercial NF membranes presented in the literature. (C)) 2012 Elsevier B.V. All rights reserved.