Chemical modification of the buried interfaces between silicon wafers and either polystyrene or poly-(methyl methacrylate) (PMMA) with the reagent (tridecafluoro-1,1,2,2-tetrahydrooctyl)dimethylchlorosilane (FDCS) was studied using liquid and supercritical carbon dioxide as the solvent and infusing agent. Above the critical point of CO2, FDCS reacts with surface silanols at the SiO2/polystyrene interface to form a monolayer, which was analyzed after removal of the polystyrene film. This is not the case below the critical point, and likely the lower diffusivity and solvent properties of the CO2-swollen film are the causes. The reaction was much less successful at the SiO2/PMMA interface. We suspect that because of strong hydrogen bonding between PMMA and the silicon substrate, modification of the SiO2/PMMA interface is achieved to only a limited extent.