A new set of experimental data on chemically equilibrium compositions for the acetic acid – n-amyl alcohol – n-amyl acetate – water systems at 318.15 K and 101.3 kPa is presented. It was found that both homogeneous and heterogeneous regions of compositions correspond to the surface of chemical equilibrium. The values of compositions were obtained by gas chromatography analysis. The critical states of liquid–liquid equilibrium were experimentally determined by the “cloud point“ technique; the chemically equilibrium critical point was located. Corresponding diagrams in 3D (the composition tetrahedron) and 2D (the square of transformed compositions variables α) concentration spaces are constructed. The NRTL model was applied to correlate liquid–liquid equilibrium data. The concentration and thermodynamic constants of chemical equilibrium were calculated using UNIFAC model.