An ultrathin polyion complex (PIC) layer containing beta-cyclodextrin (beta-CD) was formed on the surface of a charged base membrane. Thus, a positively charged copolymer containing beta-CD was synthesized by the radical copolymerization of beta-CD monomer and allylamine, and was used to modify the surface of a Nafion membrane containing negatively charged fixed ions. Electron spectroscopy for chemical analysis (ESCA) confirmed the presence of the copolymer on the surface of the Nation membrane due to the formation of a PIC layer. The ability of the membrane to separate organic isomer mixtures was tested by the pervaporation technique, using butanol isomer mixtures as the feed model. The membrane exhibited a good selectivity toward butanol isomers, indicating the effectiveness of beta-CD as a selective fixed carrier for the isomers. (C) 2003 Elsevier B.V. All rights reserved.