Liquid-liquid equilibrium data for the system heptane + o-xylene + diethylene glycol have been experimentally measured over the temperature range of 288.15 to 318.15 K. The equilibrium data of this study are analyzed using the UNIQUAC, NRTL, UNIFAC, UNIFAC-LL, and UNIFAC-DMD models as programmed by the Aspen Plus simulator. On the basis of the analyses of the experimental data of this work, UNIFAC-LL showed the best predictive performance for the mole fraction of the target species (o-xylene) in both upper (heptane-rich) and lower (DEG-rich) phases, whereas UNIFAC-DMD showed poor predictive performance in comparison with the other models.