The separation of molecules present in organic solvents by nanofiltration has potential application in several industries, and organic solvent stable nanofiltration (NF) membranes have recently become available. There is a rapidly growing body of information available on the processes controlling solvent fluxes and solute rejections in solvent nanofiltration. However, previous work has mainly been carried out with dilute solutions (< 1 wt.% solute in solvent), whereas in actual applications, solutes will be more concentrated (>5 wt.%) and phenomena such as concentration polarisation and osmotic pressure may contribute to the solvent flux, as in aqueous systems. In order to improve our understanding of organic solvent nanofiltration phenomena, experiments were performed in a cross-flow rig in which NF was carried out in a continuous mode. Solutions of different concentrations (up to 20 wt.%) of tetraoctylammonium bromide and docosane in toluene were used. Description of the experimental data, including prediction of solute rejection, was performed using the solution diffusion model for membrane transport and the film theory for liquid mass transfer effects. The results show that the organic systems cannot always be described by a simple osmotic pressure model. The flux through the membrane is affected by the cross-flow velocity, indicating that concentration polarisation induces mass transfer limitations. The fit between the model and the experimental data is markedly improved by allowing the activities of the solution components to vary, indicating that these systems are non-ideal. (C) 2004 Elsevier B.V. All rights reserved.