A Lewis acid ionic liquid, 1-[4-(chlorosulfonyl)butyl]-3-methylimidazolium chlorosulfate ([CBMIm]SO3Cl), with Lewis acid sites in both the anion and cation was synthesized. This was demonstrated to be an efficient catalyst for dehydration of aldoximes to nitrites. An unexpected self-induced phase separation of the reaction mixture was observed when the reaction was carried out in acetonitrile. This could be attributed to the hydrolysis of [CBMIm]SO3Cl to regenerate 1-(4-sulfo-butyl)-3-methylimidazolium hydrogensulfate ([SBMIm]HSO4), which was the Bronsted precursor of [CBMIm]SO3Cl.