Isobaric vapor-liquid equilibria for the binary systems acetone + methanol, acetone + 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([emim][triflate]), and methanol + [emim][triflate] as well as the vapor-liquid equilibrium for the acetone + methanol + [emim][triflate] ternary system have been obtained at 100 kPa using a recirculating still. The addition of the ionic liquid to the solvent mixture produced an important salting-out effect, and the azeotrope tended to disappear for small contents of ionic liquid. The experimental binary data sets were independently fitted with the electrolyte NRTL model, and the parameters of Mock's model were estimated for each binary system. These parameters were used to predict the ternary vapor-liquid equilibrium which agreed very well with the experimental one.