New experimental data on liquid-liquid equilibria (LLE), solubility and critical states in quaternary system propionic acid – 1-propanol – n-propyl propionate – water at 303.15 K and atmospheric pressure are presented. LLE was studied by Gas Chromatography (GC). Solubility and critical (plait) points were investigated by “cloud-point technique” method. Binodal curves and surfaces are presented in concentration tetrahedron. The critical curve is presented in 3d concentration space. A comparative analysis of the results obtained in the work with literature data is carried out. The experimental data are correlated by NRTL model. Standart deviation σ for quaternary mixture does not exceed 0.344%.