AbstractHydrophobic solvent resistant membranes were tested for the rejection of low molecular weight polymers in organic solvents with systematically varying compositions. Poly(ethylene glycols) (PEGs) of 400, 900 and 1500 Da were dissolved in solvent mixtures containing ethanol and different amounts of n-pentane, n-hexane, n-dodecane or toluene. Solvent permeabilities increased with increasing amount of alkane or toluene in mixtures with ethanol. For PEG 900 and 1500 retentions slightly decreased with increasing amount of alkane or toluene in these mixtures. For PEG 400, however, retentions increased when concentration of alkanes and toluene increased in the mixtures. The retention results were discussed in terms of the transport mechanism and the interactions between membrane material, solvents and solute.