Vapor-liquid-liquid equilibria (VLLE) data were determined for the ternary system 1-butanol + water + 2,3-butanediol at 101.3 kPa. A binary heterogeneous minimum boiling azeotrope was found at 365.95 K with mass fractions of 0.526 and 0.474 for 1-butanol and water, respectively. A modified UNIQUAC model was used to predict VLLE by using experimental data, including VLE of 1-butanol + 2,3-butanediol, VLE of water + 2,3-butanediol, LLE of 1-butanol + water, and one LLE tie-line of 1-butanol + water + 2,3-butanediol. The experimental data were compared with the calculated values. The absolute average relative deviations (AARD) are 1.65%, 1.72%, and 2.22% for organic liquid phase, aqueous liquid phase, and vapor phase, respectively. It demonstrates an appropriate fit of the modified UNIQUAC model.