The two-phase equilibrium in the system polystyrene (PS)-liquid sulfur dioxide [SO2(l)] was investigated, mainly at room temperature. The system consisted of an upper PS-rich phase, concentration (30–50)% depending on the way of preparation, and a lower phase containing less than 1% PS. The range of molecular weights of the PS samples used was 22,000–350,000 (M̄w). PS with an M̄w value of 22,000 dissolved homogeneously in SO2(l). In systems forming two phases a fractionation of PS with respect to molecular weight was observed. The M̄w value of the PS fraction dissolved in the lower PS lean phase was always substantially lower than that of the upper phase. When increasing the temperature to 90–95°C, the two phases disappeared and the PS component formed a jellylike mass with a very low solubility in SO2(l). Also in the supercritical dense SO2 gas, the solubility of PS was a few tenths of a percent only. Experiments with Soxhlet extraction indicated an extractability of a few percent.