- Abstract 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 & 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°. 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.