The separation of aqueous alcohol mixtures was carried out using a series of poly(3-alkylthiophene) membranes. The poly(3-alkylthiophene) polymers were prepared by oxidative polymerization in the presence of anhydrous FeCl3 in dry chloroform. The effect of the alkyl group structure of the poly(3-alkylthiophene) membrane on the thermal properties and the pervaporation performances were investigated. The solubility of ethanol in the poly(3-alkylthiophene) membrane is higher than that of water, however, the diffusivity of water through the membrane is higher than that of ethanol. In addition, the effects of the feed composition and the degree of swelling on the pervaporation performances of the poly(3-alkylthiophene) membranes were also studied.