The polystyrene//CO2/ethanol system has been investigated in the supercritical domain using infrared absorption (FTIR) and Raman scattering spectroscopies. The pressure and temperature were fixed in the range 20-35 MPa at 423 K for CO2/ethanol mixtures with proportion in weight from (70/30) to (58/42). These conditions are those used in our previous study concerning the fractionation of this macromolecule. A surprising finding reported before was that the CO2/ethanol mixture made of two non-solvents of PS at ambient conditions is able to dissolve PS chains under high temperature and pressure. In the present article, this question will be addressed at a microscopic level from a discussion of the intermolecular interactions between the polymer and the medium and between the CO2 and ethanol components provided by the analysis of the spectra. (C) 2001 Elsevier Science B.V. All rights reserved.