New experimental data on solubility and critical states in quaternary system propionic acid–ethanol–ethyl propionate–water at 293.15, 303.15 and 313.15 K and atmospheric pressure are presented. Solubility and critical (plait) points were obtained by “cloud-point technique” method. Experimental results on critical states in quaternary system at polythermal conditions (293.15–313.15 K) are presented as a critical surface in composition tetrahedron. The experimental data are compared with the results of calculations on the base of UNIFAC model.