The isobaric vapor-liquid equilibrium (VLE) diagram of the partially miscible binary mixture composed of -butanol and water presents a heteroazeotrope. This Demonstration computes VLE data and displays the isobaric VLE diagram as well as the boiling temperature and composition of the heteroazeotrope for values of the total pressure to be set by the user. The Non-Random Two Liquid (NRTL) model, derived by Renon and Prausnitz, is used to compute the activity coefficients in the liquid phases.