Adsorptive fouling of a polyethersulfone (PES) membrane by natural organic matter (NOM) in the presence of common metal cations was investigated with both experimental and computational molecular modeling techniques. NOM, calcium, magnesium and silicon were identified as important foulants through chemical analysis of two Midwestern surface waters. A model feed solution mimicking the lake waters and containing only NOM (Suwannee River NOM) and Ca2+ resulted in a fouling pattern similar to the surface waters; Mg2+ and Na+ caused much lower fouling at the same ionic strength as the calcium solution. Molecular modeling of the model solution allowed detailed probing of the fouling process. This work suggests that divalent ions (Ca2+ and Mg2+) may cause membrane fouling not by forming "ionic bridges" between the negatively charged functional groups on membrane surface and the negatively charged functional groups of NOM, but by promoting the aggregation of NOM molecules in solution. The carboxyl groups of NOM strongly associate with the divalent ions, while the sulfonyl groups in the polyethersulfone do not. Although Ca2+ and Mg2+ are both coordinated to the, NOM carboxyl groups predominantly by outer-sphere-type complexation, Ca2+ associates with the carboxyl groups more strongly than Mg2+ due to the looser second hydration shell structure of Ca2+. The stronger Ca2+-NOM complexation is also manifested by the decreased mobility (diffusion coefficients) of the Ca2+ bound to the NOM. (C) 2007 Elsevier B.V. All rights reserved.