Isobaric vapor liquid equilibrium (VLE) data of the reactive quaternary system methanol (1) + water (2) + methyl lactate (3) + lactic acid (4) are presented in this paper. The experimental data were correlated using the UNIQUAC model to describe the chemical and phase equilibrium. The UNIQUAC parameters for some of the binary, nonreactive systems involved in the quaternary mixture were obtained from published VLE data and, in the case of the system water + lactic acid, from experimental VLE data reported in this paper. The rest of the binary UNIQUAC parameters were obtained by an optimization of the correlation of the experimental quaternary VLE data. The results obtained from the correlation were found to be in good agreement with experimental data. The reaction equilibrium constant was also calculated for each experimental data point. A three-dimensional phase diagram was constructed for the quaternary reactive system by using transformed composition variables. No reactive azeotrope was found.