The thickness and refractive index of poly(dimethylsiloxane) (PDMS) thin films an silicon, exposed to high-pressure carbon dioxide, have been measured in-situ by spectroscopic ellipsometry. The swelling of the film is determined from the thickness and the sorption of CO2 from the refractive index. The sorption and swelling Values for the thin films exceed those of the bulk films. These increases may be attributed to excess CO2 at the polymer thin film-CO2 and polymer-silica interfaces and the influence of the silica surface and CO2 on the orientation of the polymer. Pressure-induced birefringence of the fused-silica windows was addressed in the model calculations and corrections were found to be successful for a thermal oxide reference wafer and the PDMS films. Large surface excesses of CO2 on the reference wafer were observed in regions where CO2 is highly compressible due to critical adsorption.