AbstractAn equation of state, suggested previously, has been extended and applied to calculate phase equilibria and critical phenomena of ternary and quaternary fluid systems to high temperatures and pressures. The equation consists of a modified hard sphere repulsion term and an additive perturbation term based on a square well potential to account for molecular attraction. The procedure of calculation is described. It uses exclusively binary interaction parameters, derived from properties of the binary sub‐systems. — In order to compare the calculated results, phase diagrams have been experimentally determined for the system H2O—N2‐hexane to 679 K and 270 MPa. The experimental procedure is described. The comparison is made with isothermal and isobaric prismatic diagrams. The agreement is satisfactory above 500 K. Results of calculations are also shown for the ternary systems H2O—N2—CO2and H2O—CO2‐hexane as well as for the quaternary system H2