Nanotiltration experiments in methanol and ethanol were carried out for six reference components with different molecular weights (MW 228-880) and polarities (log P 0-12). The contribution of diffusion to solute transport, calculations based on results from cell diffusion experiments, was found to be only 1-7%; solute transport occurs mainly by convection. Furthermore, it was found that solute transport is influenced by solute-solvent-membrane interactions. Solvent-solute interactions (solvation) cause a different effective solute diameter in each solvent: it is smaller in ethanol than in methanol, resulting in lower rejections in ethanol than in methanol. Solute rejection increases with increasing molecular size (for components with similar polarity). Solute-membrane interactions were expressed in polarity terms and charge effects. A decrease of the rejection with decreasing solute polarity (for components with similar MW) was observed. Since non-polar components (high log P) are exposed to smaller repulsion forces from the polymeric membrane material, the resistance against solute permeation is lower for these components. The solvent-membrane interactions were found to result in solvation of the pore wall; the degree of membrane solvation is different for each solvent. It is determined by the affinity between the solvent and the membrane, and by the molecular size of the solvent. In ethanol, hydrophilic membranes show a larger drop in solute rejection than hydrophobic membranes. The differences in solvent-membrane affinity (measured by contact angle) are much smaller for the first membranes, and therefore pore wall solvation decreases with increasing solvent size. Hydrophobic membranes have a much larger affinity for ethanol than for methanol, leading to stronger interactions, but undergo competitive forces due to the larger solvent size. Therefore, the difference in degree of solvation and effective pore diameter is less pronounced. Based on these three observed or postulated interactions, rejections of all six reference solutes in methanol and ethanol could be explained. (c) 2005 Society of Chemical Industry.