The associating lattice cluster theory (ALCT) is developed in this work. It is especially useful for describing mixtures of oligomers of varying architecture. As the influence of the structure is predicted, it can also be applied in cases in which pure component data are lacking. ALCT is used here for the calculation of liquid-liquid equilibria (LLE) in the system oleic acid (OA)+formic acid (FA)+formoxystearic acid that is interesting for biopolymer production. Experimental studies of LLE in that system were carried out at 296, 313, and 333K. Ternary mixtures were prepared from the educts OA and FA using heterogeneous acidic catalysis. Quantitative NMR spectroscopy was applied for the analysis. For the educt system also the excess enthalpy was studied by isothermal titration calorimetry. The prediction of the ternary LLE by ALCT succeeds. (c) 2018 American Institute of Chemical Engineers AIChE J, 65: 783-791, 2019