Isobaric vapor-liquid equilibrium (VLE) data for methanol-methyl ethyl ketone (MEK) systems containing ionic liquids (ILs) have been measured with a modified Othmer still at atmospheric pressure (101.32 kPa), and they were correlated by the NRTL equation. Three ILs, composed of an anion tetrafluoroborate ([BF4](-)) and a cation from 1-ethyl-3-methylimidazolium ([EMIM](+)), over 1-butyl-3-methylimidazolium ([BMIM](+)), to 1-octyl-3-methylimidazolium ([OMIM](+)), were investigated, and they all gave rise to a change of the relative volatility of methanol to MEK. The results indicated that, among the three ILs studied, [BMIM](+)[BF4](-) and [OMIM](+)[BF4](-) eliminated the azeotropic point at mole fraction 30 % and 10 %, respectively, whereas IL [EMIM](+)[BF4](-) pulled down the azeotropic point. The influence of the variation of the cation's alkyl chain length of imidazolium tetrafluoroborate-based ILs on VLE of the azeotropic system methanol-MEK was discussed.