Solubility, critical states and liquid-liquid equilibrium (LLE) in the quaternary reacting system formic acid – ethanol – ethyl formate – water and in the ternary subsystems formic acid – ethyl formate – water and ethanol – ethyl formate – water were studied experimentally at 298.15 K and 308.15 K and atmospheric pressure. Binodal curves, binodal surfaces, tie-lines and critical manifolds were determined. In order to construct binodal surfaces and critical curves of the quaternary system, five cutting planes with several different ratios of concentration of formic acid to ethanol were studied. Experimental LLE data were compared with the data calculated by NRTL model, and it was found that experimental and calculated data are in a good agreement.