The focus of this work is physico-chemical study of model mixtures containing butanol – a component of perspective biofuel. New detailed data on liquid-liquid equilibria (LLE), solubility and critical compositions were obtained at 293.15 K and atmospheric pressure for quaternary propionic acid – n-butanol – n-butyl propionate – water system. Research methods: gas chromatography, “cloud-point technique”. A brief review of the available data on the mutual solubility and LLE in this system is presented. The regularities of phase diagrams evolution with temperature changes are discussed. The compositions of critical phases were determined at 293.15 and 303.15 K. These data gave opportunity for the construction of polythermal critical surface in a 3D composition space. Correlation of LLE results with the use of NRTL indicates sufficient agreement with the calculated data.