Organic nanofiltration membranes behave differently in water-alcohol solutions and in aqueous solutions. This study analyzes the permeation of organic solutes in aqueous and hydroalcoholic solutions through NF membranes. All experiments were conducted in a batch operation in presence of NaCl at pH =3.5. First, the solvent permeability constant of ethanol-water mixtures through various reverse osmosis membranes was analyzed. With all membranes tested, the solvent permeability decreased significantly with the increase in feed ethanol volume fraction. The rejections of organic solutes tested were high (>93%) in the absence of ethanol, and fell significantly (to <60%) in its presence. Therefore, membrane behavior was modified by the presence of ethanol, due possibly to the reduction in the polarity and dielectric constant of the solvent and the swelling of the membrane. The experimental data fitted satisfactorily to the extended Spiegler-Kedem model for multicomponent systems. In this model, the solute rejection depends on the solute permeability and on the overall solute-solute permeability constants. (C) 2012 Elsevier B.V. All rights reserved.