Nanofiltration polyethersulfone membrane has been modified by means of UV-initiated graft polymerization technique using the weakly acidic monomer acrylic acid and the immersion method. Different UV-irradiation times and acrylic acid concentrations in water were studied. Both the unmodified and the modified polyethersulfone nanofiltration membranes were characterized by attenuated total reflection-Fourier transform infrared spectra (FTIR-ATR) and atomic force microscopy (AFM). Irreversible membrane fouling has been studied by using humic acid model solutions at two different pH values (7 and 3). It was observed that some modified membranes exhibited higher permeance than the unmodified polyethersulfone nanofiltration membrane. The humic acid rejection factor was higher for all modified membranes compared to the unmodified membrane and the irreversible fouling by humic acid molecules was reduced after UV-initiated graft polymerization by acrylic acid. (C) 2010 Elsevier B.V. All rights reserved.