The water flux for two nanofiltration membranes (UTC-20 and NF70) was measured using aqueous solutions of 11 organic compounds in different concentrations. The flux declined, compared to the pure water flux, by more than 50% for solutions containing less than I g/l of some organic compounds. Flux decline as a function of the concentration of the organic compound showed the typical shape of a Freundlich isotherm. Comparison to adsorption experiments indicated that flux decline, for the solutions used, was related to adsorption on the membrane material. The dipole moment, the octanol-water partition coefficient and the water solubility were examined as parameters to explain adsorption on nanofiltration membranes. A clear correlation was found between the octanol-water partition coefficient and adsorption; adsorption also appeared to be related to the dipole moment and the water solubility. This shows that both the surface charge and hydrophobicity of the membrane can play a role in the adsorption. When flux decline is related to octanol-water partition coefficient, the molecular size (molecular weight) should be taken into account as well. A subset of molecules with a molecular weight in a narrow range, somewhat below the cut off value of the membranes, was selected. Within this subset, a clear correlation between the octanol-water partition coefficient and flux decline was observed. (C) 2002 Elsevier Science B.V. All rights reserved.