This study was carried out to investigate the effect of monovalent Na+ and divalent Ca2+ on UF membrane fouling behavior of different NOM fractions humic substances, proteins and polysaccharides, as represented by humic acid (HA), bovine serum albumin (BSA) and dextran (DEX), respectively. The results showed that Na+ was able to reduce the membrane fouling by HA and BSA; while Ca2+ increased both the total fouling resistance and fouling irreversibility significantly, illustrating the different effect of monovalent and divalent cations on membrane fouling by negatively-charged NOM fractions. On the other hand, for the nearly-neutral DEX, both Na+ and Ca2+ exhibited a slight reduction effect on the membrane fouling. The effect of Na+ and Ca2+ on UF membrane fouling by combination of different NOM fractions (e.g. HA-BSA and HA-DEX) was also examined in comparison with that by HA alone. As for the HA-BSA combination, synergistic fouling effect was observed in the presence of Na+; however, when Ca2+ was added (no matter with or without Na+), the fouling by HA-BSA was decreased as compared with HA alone. As for the combination of HA-DEX, no matter Ca2+ was present or not, monovalent Na+ promoted more severe membrane fouling for the HA-DEX in comparison with HA alone; only when Ca2+ was added alone, a reduced fouling was obtained by the HA-DEX. Furthermore, it was also observed that in the combination system, the fouling irreversibility was generally lower than that of HA alone, maybe due to the BSA and DEX molecules could interfere with the strong interactions of HA-membrane and HA-HA. (C) 2013 Elsevier B.V. All rights reserved.