A UV-assisted photochemical graft polymerization technique was used to produce modified poly(ether sulfone) (PES) ultrafiltration (UF) membranes that exhibit reduced interaction with natural organic matter (NOM), as a route to reduce the fouling caused by NOM. We evaluated six different hydrophilic monomers for their ability to reduce fouling by NOM: two are neutral monomers, N-vinyl-2-pyrrolidinone (NVP) and 2-hydroxyethyl methacrylate (HEMA); two are weakly acidic (carboxylic) monomers, acrylic acid (AA) and 2-acrylamidoglycolic acid (AAG); and two are strongly acidic (sulfonic) monomers, 3-sulfopropyl methacrylate (SPMA) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS).Grafting increased membrane surface wettability and shifted the membrane pore size distribution to smaller sizes, which increased natural organic matter rejection (except in the case of NVP). Total fouling appeared to depend primarily on solute rejection, and varied in a complex way that could be interpreted in the context of the NOM molecular weight distribution. Reversible fouling resulting from cake formation was only weakly dependent on membrane surface chemistry; in contrast, irreversible fouling exhibited a marked dependence on surface chemistry. Membranes modified with the weak acid AA monomer were able to reduce irreversible fouling to zero, in contrast to other strongly hydrophilic monomers such as HEMA and AAG, which increased irreversible fouling relative to the unmodified membrane. We conclude that wettability (or hydrophilicity) is not an appropriate parameter for estimating reduced fouling potential for NOM feeds, as it is for feeds containing protein. We hypothesize that this is a consequence of the structural and chemical heterogeneity of NOM. The AA-modified membranes exhibited excellent filtration performance over multiple runs, and outperformed a regenerated cellulose (RC) membrane having similar initial NOM rejection. (C) 2003 Elsevier Science B.V. All rights reserved.