The sorption characteristics of water, methanol and ethanol vapours in cellulose acetate (CA) Ðlms were measured by microgravimetry. The sorption isotherms for water vapours in the CA Ðlm at di †erent temperatures in the range 20È40¡C do not obey the Flory equation over the whole water activity range. At high water activities, the sorption extent increases much faster with the water activity than it should according to the Flory approach. The isotherms over the whole water activity range can be Ðtted well by the ENSIC model, a new mechanistic model developed to account for the possibility of solvent cluster formation in the polymer material. Similar behaviour was observed for ethanol, which shows a lower tendency to form clusters, but higher affinity to CA. The sigmoidal shape found for the methanol sorption isotherm suggests a strong sorption on CA sites at low methanol activities. The GuggenheimÈAndersonÈDe Boer equation Ðtted well this isotherm. The di †usion coefficient, which was calculated from Fickian sorption kinetics at di †erent solvent activities by curve Ðtting, was constant for water but increased with ethanol content in the membrane according to an exponential relationship characterized by a limit di †usion coefficient and a plasticization coefficient. The limit di †usion coefficient for water was two orders of magnitude larger than that for ethanol, but the activation energy for ethanol was twice as large. Methanol di †usion was only Fickian at a low solvent activity ; the di †usion coefficient was one order of magnitude lower than that for water.