Fouling in nanofiltration was studied using model substances similar to those in the pulp and paper mill waters, such as vanillin, humic acid, locust bean gum, and karaya gum. The parameters studied were the concentration of foulants, pH, cross-flow velocity and membrane hydrophobicity. The substances were nanofiltered alone and in their binary mixtures. The nanofiltration membranes (NTR-7450 and Desal-5 DK) differed by their cut-off values and their material properties, such as hydrophobicity. Various causes of fouling were observed to exist with the model substances studied. The electrostatic attraction markedly influenced the flux of charged substances, but had an insignificant effect on the flux of uncharged substances. Hydrophobic interactions overcame the electrostatic repulsion and caused more fouling of the more hydrophobic membrane. The prediction of filtration properties of the binary mixtures of model substances from the filtration results of individual substances is extremely difficult. Both the interactions between two model substances as well as the interactions between the model substances and the membrane determined the flux and fouling in the filtration of the binary mixtures of the model substances. Humic acid improved both the permeate flux and the pure water flux at neutral pH, but slightly fouled the membranes under acidic conditions. Humic acid might increase both the membrane hydrophilicity and its negative charge when being adsorbed by hydrophobic attraction. (C)2000 Elsevier Science B.V. All rights reserved.