Supercritical fluid chromatography is a powerful technique for analysis, and it also provides a useful tool for the measurement of thermodynamic properties. For any application, the method involves equilibration between the mobile supercritical fluid phase and the polymeric stationary phase. The mobile-phase properties ale often modified substantially with the addition of polar or protic cosolvents, but these adducts also cause changes in the stationary phases. In situ FTIR spectroscopy and the swelling of poly(dimethylsiloxane) (PDMS) are combined to quantify these changes due to partitioning of the cosolvent into and the swelling of the polymeric stationary phase as a function of fluid pressure. These results are used to evaluate the magnitude of the corrections for the changes in retention time of analytes, which affects both analytical methods and thermophysical property measurement.