Chemical equilibrium (CE) in the quaternary reacting system formic acid-ethanol-ethyl formate-water was experimentally studied at 298.15 K and atmospheric pressure. The CE compositions were determined by gas chromatography analysis. The obtained data gave an opportunity to present the disposition of the surface of CE in a composition tetrahedron. The constants of CE "concentration" and thermodynamic) were determined on the base of experimental data and UNIFAC model.