Experimental data on solubility, critical states and liquid–liquid equilibrium (LLE) for the quaternary system acetic acid – n-butanol – n-butyl acetate – water and ternary subsystems acetic acid – n-butanol – water, n-butanol – n-butyl acetate – water and acetic acid – n-butyl acetate – water were obtained at 318.15 K and at atmospheric pressure. The solubility and critical states were studied by isothermal titration method (“cloud-point technique”). The LLE was determined using gas chromatographic analysis. Binodal surface in quaternary system and binodal curves in ternary systems were constructed. Compositions of critical points (plait points) and critical curve were determined. The NRTL model was applied to calculate the LLE data for the quaternary and ternary systems. The results of NRTL modeling are in good agreement with the experimental data.