Thermodynamic modeling of equilibria in reacting systems was studied using systems with ester synthesis reaction as an example of multicomponent system with liquid phase splitting. The processes in these systems include several stages, i.e., one or few phase transitions (liquid- liquid, vapor- liquid and chemical reaction. Chemical equilibrium in quaternary reacting systems with ester synthesis reaction (acetic acid-n-propanol- n-propyl acetate-water and acetic acid-ethanol-ethyl acetate-water) was calculated. The results were in good agreement with the experimental data sets on LLE. Model calculations for all ternary subsystems with miscibility gap were carried out. The results confirmed the potentialities of the use UNIFAC model for robust approximation LLE data in this type of reactive systems. This is an abstract of a paper presented at the CHISA 2012 - 20th International Congress of Chemical and Process Engineering and PRES 2012 - 15th Conference PRES (Prague, Czech Republic 8/25-29/2012).