The use of ionic liquids (ILs) in chemistry and electrochemistry has been studied for some decades. Only recently, researchers have focused on the suitability of ILs as selective solvents in the field of separation technology. ILs represent suitable entrainers and extraction solvents for the separation of azeotropic mixtures by means of extractive distillation and solvent extraction. In this work, ternary vapor-liquid and liquid-liquid equilibria of the azeotropic mixtures ethanol + water and THF + water containing different kinds of commercially available ILs are presented. The four ionic liquids used are derived from butyl-methylimidazolium tetrafluoroborate [BMIM](+) [BF4](-) by systematic variation of the cation or the anion. The influence of the IL structure on the relative volatility of the low boiling component in extractive distillation and on the selectivity in solvent extraction processes is discussed in this work.