Consistent vapour-liquid (VLE) and vapour-liquid-liquid (VLLE) data for the ternary system water (1) + ethanol (2) + n-heptane (3) are reported at 101.3 kPa at temperatures in the range of 341-352 K. An all-glass, dynamic recirculating still equipped with an ultrasonic homogenizer has been used in the determination of the VLLE. The experimental data indicates that a ternary heterogeneous azeotrope is present at 341.83 K with a composition of 0.205, 0.432, and 0.363 mole fractions of water, ethanol, and heptane, respectively. The experimental results have been used to test the accuracy of the UNIFAC, UNIQUAC, and NRTL models in the calculation and prediction of the equilibrium data. None of these methods properly correlate and approximate the top zone of the non-isothermal solubility curve, which is important for the simulation of the azeotropic distillation. (c) 2006 Elsevier B.V. All rights reserved.