In this paper, a novel method was proposed to synthesize ionic liquids by using a specially designed four-compartment configuration electrodialyzer. 1-Butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF4), used as a model ionic liquid, was successfully synthesized and concentrated. The products of [BMIM]BF4 could be easily obtained by using the permselectivity of ion-exchange membranes and the migration of ions under dc electric field. The structure of the products was identified with Fourier transform spectra (FTIR) and H-1 NMR. It was noted that 92% yield ratio could be achieved while the purity was over 95% after operating 1 h under the voltage of 10 V. (C) 2008 Elsevier B.V. All rights reserved.