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We investigated the isobaric vapor-liquid equilibria (VLE) of the binary system methanol + tetrahydrofuran (THF), the ternary system methanol + THF + 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMIM][NTf 2 ]), as well as methanol + THF + 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([HMIM][NTf 2 ]) in a modified Othmer still. The results indicated that the ionic liquids ([BMIM][NTf 2 ] and [HMIM][NTf 2 ]) produced a consistent salting-in effect of THF, resulting in the incremental relative volatility of methanol to THF. The experimental data were correlated using the e-NRTL model, and the parameters were obtained with a good consistency. The ionic liquids even eliminated the azeotropic phenomenon when the mole ratio of [BMIM][NTf 2 ] was up to 0.173 and [HMIM][NTf 2 ] up to 0.128 as calculated by the e-NRTL model. [HMIM][NTf 2 ] had a stronger effect on the separation of azeotropic mixtures than [BMIM][NTf 2 ] did. [ABSTRACT FROM AUTHOR]