The pervaporation separation of the MeOH/MTBE mixture was performed with three cellulose acetate membranes under various feed compositions and operation temperatures. The results show that the acetylation degree has great influence on the pervaporation properties because the quantity of hydroxyl groups in the polymer matrix leads to different crystallinty degrees. The different extent of the plasticization effect resulted in membranes performing their special flux trend and selectivity over the range of feed compositions and caused the permeants to transport through the membrane in different diffusion modes under various operation temperatures. The IR sorption spectra evidenced that for membranes with different acetylation degrees there existed different sorption sites for penetrants and provided a proof for different pervaporation properties of the tested membranes. (C)2000 Elsevier Science B.V. All rights reserved.