AbstractThe effect of supercritical ethane on the high‐pressure phase behavior of the polystyrene‐toluene system is experimentally investigated. Adding 17.8 wt. % ethane to the polystyrene‐toluene solution lowers the temperature of the lower critical solution temperature (LCST) curve by 162°C, 22.5 wt. % lowers the temperature of the curve by 231°C, and with 24.9 wt. % the curve is shifted to such low temperatures that it merges with the upper critical solution temperature curve. In practice this means that polymer solutions can be separated at lower temperatures where thermal degradation of the polymer is less. Also, more than 99 wt. % of the polystyrene is recovered in the polymer‐rich phase after the solution phase splits. Patterson's corresponding states treatment is used to model the