The objective of this study is the investigation of solubility, critical states and liquid-liquid equilibrium of the quaternary system oleic acid – methanol – methyl oleate – water and its ternary and binary subsystems at 303.15 K and atmospheric pressure. A brief review of the literature on the studied system is presented. The compositions for binodal curves and for surface of solubility were determined using cloud-point technique. Tie-lines of liquid–liquid equilibrium were obtained using NMR spectroscopy. Experimental results were represented by diagrams in concentration simplices including 3D phase diagram in composition tetrahedron. A comparative analysis with the available literature data has been carried out. Correlation and prediction of experimental LLE data was performed using NRTL and UNIFAC models, respectively. Results of modeling was found to has a satisfactory agreement with the experimental data.