The measurement of sorption and diffusion behavior of liquid ethanol and water solvent mixtures in polyurethane membrane were made simultaneously by thermogravimetry. The individual amounts of sorbed water and ethanol in the polymer membrane were estimated by thermogravimetry and differentiated by mass spectrometry. In addition, from a single dynamic thermogravimetric experiment the activation energy for solvent molecules desorbing from the polymer membrane was also determined. The thermodynamic activity of ethanol vapor in equilibrium with the ethanol-water-polyurethane system was determined by gas-liquid chromatography. The clustering functions, the mean numbers of solvent molecules in the clusters, and those associating with polymer sites were evaluated by applying simplified mathematical derivatives using the experimentally determined values of activity and volume fraction of solvent molecules. It was found that at lower ethanol concentration the tendency for ethanol molecules to cluster together is high. At higher ethanol activity, ethanol-polymer site interactions predominantly occurred.