Surfactants are present in almost all aqueous solutions-either as additives for different purposes, or because they occur naturally. Because of the common occurrence of surfactants in process water it is important to know how they behave in membrane processes. Ultrafiltration membranes allow almost complete passage of surfactant monomers, but reject micelles almost completely. Concentration polarisation during ultrafiltration of surfactant solutions is therefore mainly influenced by the presence of micelles. Operating parameters, e.g. the transmembrane pressure and the concentration of surfactant, as well as the pure water flux of the membrane, have a marked influence on the performance of hydrophilic membranes, as shown in this investigation. A distinct difference between the interaction of a non-ionic surfactant with hydrophilic and hydrophobic membranes was observed. The hydrophobic membrane showed a flux reduction already at concentrations below the critical micelle concentration (CMC), whereas no flux reduction was observed for a hydrophilic membrane with the same nominal molecular weight cut-off, below the CMC.