New experimental data on liquid-liquid equilibrium (LLE) and critical phases for the propionic acid – n-butanol – n-butyl propionate – water system at 303.15 K and atmospheric pressure are presented. The LLE compositions were determined by gas chromatography, correlated and predicted by NRTL and UNIFAC models respectively. The results of NRTL and UNIFAC modelling are in sufficient agreement with experimental data. Standard deviations for binary and ternary subsystems do not exceed 0.79% (NRTL) and 5.1% (UNIFAC). In case of quaternary system investigated σ(NRTL) = 0.56% and σ(UNIFAC) = 2.2%. The study of critical states was carried out by “cloud-point technique” method. The run of critical curve in quaternary system was studied.