BACKGROUND: tert-Butanol (TBA) and water form an azeotrope at atmospheric pressure. Therefore, extractive distillation was adopted to separate the azeotropic mixture. As a new kind of solvent, the ionic liquids (ILs) 1,3-dimethylimidazolium dimethylphosphate ([MMIM][DMP]) and 1-ethyl-3-methylimidazolium diethylphosphate ([EMIM][DEP]) were chosen to break the azeotropic point in extractive distillation. RESULTS: For the ternary mixture of TBA + water + ILs, the isobaric vapor-liquid equilibrium data at atmospheric pressure for three IL mole fractions (0.1, 0.2 and 0.3) were measured. The results indicated that the volatility of TBA relative to water became higher after [MMIM][DMP] or [EMIM][DEP] was added. The nonrandom two-liquid (NRTL) model and the COSMO-RS theory were used to predict the ternary experimental data. The predicted results from the NRTL model showed good agreement with the experimental data. The predicted results from COSMO-RS showed that the vapor-phase mole fraction basically agreed with the experimental data and the equilibrium temperatures were lower than the experimental data. ⊞-Profiles, ⊞-potentials and ⊞-surfaces were calculated based on COSMO-RS. The results were the same as for the experiments, namely the interaction between water and ILs was stronger than other interactions; the affinity of water and [MMIM][DMP] was stronger than that of water and [EMIM][DEP].