Isothermal pressure elevation can sometimes cause liquid-liquid phase separation of binary liquid mixtures. A quantitative thermodynamic analysis of this effect is made and applied to the system acetone–carbon disulfide at 0°C. with the use of available P-V-T-X data and vapor pressure data at low pressure. Visual observations of the phase separations at pressures up to 80,000 lb./sq.in. were used to compare with the results of the thermodynamic analysis.