This paper reports the flux, retention and stability of nanofiltration (NF) membranes with aqueous and organic solvents, and three selected solutes having the same molecular weight of 350 Da: Orange II, Safranine O and Solvent Blue 35. It appears that the nominal manufacturer-specified molecular weight cut-off is only valid for predicting the rejection in aqueous solutions, and that the retention in organic solvents either for molecules of the same molecular weight or even for the same molecule is unpredictable and depends on the specific solvent. It was found that solute retention in organic solvents is lower than in aqueous solution, and that the solvent has significant influence on the retention. These interesting results were not due to membrane deterioration in the presence of organic solvents for any of the membranes studied in this work.