Waveguide spectroscopy has been applied to optically study the influence of the concentration (0-1.5 M) of methylmethacrylate (MMA) in supercritical CO2 on the refractive index and swelling of PMMA as a function of pressure (up to 52 MPa at 333 K). The refractive index of pure CO2 is compared to literature in order to validate the technique in a supercritical fluid environment. The MMA content was varied, which resulted in an increased refractive index of the fluid in conjunction with a decreased refractive index of the PMMA film. This behavior is a direct result of a larger sorption and therefore swelling of the polymer layer and was measured at different pressures. With increasing pressure an increase of the refractive index of the solution and a decrease of the one of the PMMA layer was observed. The obtained refractive indices were modeled assuming an ideal mixture. The comparison with the experimental results showed that the qualitative behavior agrees well with the calculations.