Aromatic copolyester of poly(trimethylene-co-ethylene terephthalate) (PTET) with different composition was synthesized and the PTET sample with 60% weight fraction of polytrimethylene (PTET-60) was amorphous. The compatibility of PTET-60/cellose acetate (CA) blends and the pervaporation of their membranes for separation of benzene/cyclohexane mixtures were investigated. It was found that PTET-60 is compatible with CA when the weight fraction of PTET-60 in PTET-60/CA blends (WPTET-60) is lower than 0.35 and more than 0.5. Both the degree of swelling (DS) and the permeation flux (J) of these blend membranes increased with increasing WPTET-60 from 0 to 0.35, and a maximum value of the separation factor (alpha) displayed at WPTET-60 = 0.25. (c) 2006 Wiley Periodicals, Inc.