In this study, fouling-resistant nanocomposite membranes were prepared via electrostatic self-assembly between TiO2 nanoparticles and sulfonic acid groups on the membrane surface. Sulfonated polyethersulfone (SPES) was prepared by heterogeneous sulfonation with chlorosulfonic acid, and a TiO2 nanoparticle solution was prepared from the controlled hydrolysis of titanium tetraisopropoxide. Then, the SPES ultrafiltration membranes, which were prepared by the phase inversion method, were dipped in the TiO2 nanoparticle solution, thereby producing the TiO2/polymer nanocomposite membranes. The fouling mitigation property of the nanocomposite membrane was examined using membrane bioreactor (MBR) sludge, which contains a great number of various foulants. The results showed that the amount and rate of membrane fouling decreased in the nanocomposite membranes compared to the polymeric membranes. (c) 2006 Elsevier B.V. All rights reserved.