The hydrophilic/hydrophobic properties of the membrane material have an important influence on the membrane flux reduction when treating solutions containing hydrophobic solutes. Different methods of measuring the hydrophobic properties of membranes have been evaluated; measurement of the contact angle being the most commonly used method. In this investigation the flux reduction of eight membranes of approximately the same cut-off, but of different materials, was determined and related to the contact angle. A low-molecular weight hydrophobic solute (octanoic acid) was used as foulant. The flux of the hydrophilic membrane was only marginally reduced, whereas the flux reduction of hydrophobic membranes was significant. Membranes of polyvinylidene fluoride had the lowest flux reduction of the hydrophobic membranes. Differences between membranes of the same material (polyethersulfone), but from different manufacturers, were observed. No relation was found between the flux reduction and the contact angle of the membrane, the initial pure water flux or the retention of a hydrophilic macromolecule (PEG20). The pure water flux of two of the hydrophobic membranes increased significantly during the experiments, whereas the pure water flux of the other hydrophobic membranes decreased. A membrane of polyaramide, with a contact angle of the same order of magnitude as the other hydrophobic membranes, behaved as a hydrophilic membrane, with no influence on the flux at concentrations below the solubility limit of the hydrophobic solute, but experienced the most severe flux decline of all the membranes included in the investigation at the solubility limit. Two modified membranes were included in the investigation. The flux reduction of these membranes was not significantly lower than for the corresponding unmodified membranes of the same materials.