The fouling behavior of humic substances was investigated based on studies of the membrane-solute- (adsorptive fouling) and membrane-solute-solute (dynamic fouling) interactions. Furthermore, the surface of ultrafiltration (UF) membranes made of polyethersulfone (PES) was modified by photograft copolymn. with poly(ethylene glycol)methacrylate (PEGMA). The aim of the surface modification was to obtain a thin-layer, polymer, hydrogel, PES-based UF composite membrane with high permeability and selectivity and a low fouling tendency. The modified membranes were more hydrophilic than the com. PES membranes. Ultrafiltration expts. were performed with acidic and alk. solns. of humic acid in the presence or absence of Ca2+ ions. In fact compared to com. PES membranes, the fouling tendency was reduced with respect to adsorptive as well as dynamic fouling.