It is well-known that liquid ethanol at ambient conditions is a nonsolvent for polystyrene (PS), Surprisingly, the supercritical(SC) CO2/ethanol mixture is able to solvate PS, allowing the use of such media to perform fractionation of PS.(1) The first part of this work is devoted to the determination of the solubility of PS in the SC CO2/ethanol solution and to assess the selectivity of the mixture using UV spectroscopy. The second part concerns an infrared absorption and Raman scattering study aimed at the understanding of the microscopic interactions involved in this system. We have performed measurements on CO2/ethanol mixtures at various compositions (weight % of ethanol = 10, 30, and 42) in the temperature range 333-423 K and at pressures ranging from P = 15 up to 35 MPa. Several species were found to coexist, namely, ethanol aggregates and monomers. In addition, evidence of a weak interaction between CO2 and ethanol was found. Finally, monomeric ethanol is likely to be responsible for the PS solvation in the SC CO2/ethanol mixture.