In this study the influence of a low-molecular hydrophobic solute on the Bur of a hydrophilic regenerated cellulose membrane and hydrophobic polysulphone membranes with different nominal molecular weight cut-off, NMWCO, was investigated. Octanoic acid was used as a model substance because it is considerably smaller than the NMWCO of all the membranes used in the investigation. Octanoic acid was found to decrease the flux of the polysulphone membranes considerably. The decrease was dependent on the NMCWO, although in all cases the NMWCO is many times larger than the size of the octanoic acid molecules. It is also shown that the Bur of the hydrophilic membrane, with a NMWCO of the same size as one of the denser hydrophobic membranes, is unaffected by the octanoic acid. The cross-sections of both used and unused membranes were examined in a scanning electron microscope (SEM). No visual difference was observed between the used and unused membrane samples. Analysis of the membranes with Fourier transform infrared spectroscopy (FTIR) showed that octanoic acid remained in the membranes even after thorough cleaning.