Long-term stability and performance of polymeric membranes in solvent and mixed solvent media can be reduced due to sorption and swelling of the membrane matrix. For this reason quantification of sorption and swelling is of major importance for the development of future applications of membrane processes in solvent and mixed solvent media. In this work a method is discussed, based on attenuated total reflectance infrared spectroscopy (ATR-IR), to establish sorption and sorption selectivity of a cellulose acetate (CA) membrane in water/methanol and water/ethanol mixtures. By analysis of specific peaks from the ATR-IR spectra of the solvents, the preferential sorption of water in CA membranes can be quantified. In the presence of methanol, the selectivity for water ranges from 2.5 to 3.5 between 52 and 90% of methanol. For ethanol, the selectivity for water ranges from about 1 (30% ethanol) to 2 (90% ethanol). From the work it follows that ATR-IR provides an easy and non-destructive method to study the sorption behavior of the polymeric membrane separation layer.