This paper describes a method for the determination of the relationship between solute molecular weight and rejection, often referred to as the molecular weight cut off (MWCO) of the organic solvent nanofiltration (OSN) membranes. This method utilises a homologous series of styrene oligomers spanning the nanofiltration range (200-1000 g mol(-1)). Good separation of the individual oligomers was achieved using liquid chromatography with a reverse phase C 18 column. The good solubility of the styrene oligomers in many organic solvents makes cross comparison of membrane performance across these solvents possible. This method also allows the determination of the molecular weight at which a rejection of > 99.9% occurs in different solvents, making it a useful tool to test membrane and equipment integrity. (c) 2007 Elsevier B.V. All rights reserved.