Vapour-liquid equilibrium (VLE) accompanied with chemical reaction in the quaternary water + methanol + acetic acid + methyl acetate system was measured in a modified Dvorak and Boublik still; 20 experimental points at 323.15 K were determined. For the complete and consistent description, the isothermal VLE data in the four non-reacting binary subsystems were measured at the same temperature. The data were correlated by means of the maximum likelihood procedure by using the NRTL model; binary model parameters for the binary subsystems were obtained from experimental vapour-liquid equilibrium data by using either the Hayden-O'Connell or Marek-Standart correction as appropriate, to account for the non-ideality of vapour phase. NRTL parameters for the reacting binary subsystems were estimated from the global optimization based on the quaternary experimental data. The results obtained from the correlation show a good agreement of calculated and experimental data. By using transformed composition variables, two-dimensional phase diagrams for the quaternary system have been constructed. (c) 2006 Elsevier B.V. All rights reserved.