Acetic acid and N,N-dimethylacetamide (DMAC) form a binary maximum boiling azeotrope, and the azeotrope can be separated by extractive distillation. In this work, 1-butyl-3-methylimidazolium bis [(trifluoromethyl)sulfonyl]-imide ([BMIM][Tf2N]) was selected as solvent to separate the azeotrope, and isobaric vapor-liquid equilibrium (VLE) data of the system acetic acid + DMAC [BMIM][Tf2N] were determined at 7.0 kPa and 12.0 kPa. The non-ideal behavior in vapor phase was caused by the associating effect of acetic acid and calculated by chemical theory and Hayden-O'Connell equation. The experimental VLE data were correlated by the nonrandom two-liquid (NRTL) activity coefficient model and the correlated results agreed well with the experimental data. The results show that ionic liquid [BMIM] [Tf2N] produces a notable salting-out effect in the azeotropic mixture, which enhances the relative volatility of acetic acid to N,N-dimethylacetamide. (C) 2015 Elsevier B.V. All rights reserved.