Carbon dioxide produces an anomalous maximum in the swelling of poly(methyl methacrylate) (PMMA) thin films that is not present in bulk films, as shown with in-situ spectroscopic ellipsometry, This maximum and a corresponding minimum in refractive index are observed in regions of pressure where CO2 is highly compressible near the critical point. An effective excess thickness, determined from the height of the anomalous swelling maxima, increases proportionally with increasing initial film thickness, h(0), in the range studied of 85 nm < h(0) < 325 nm. Therefore, the anomalous swelling maxima suggest concentration inhomogeneities in the thin film, i.e., the onset of polymer/CO2 phase separation, that are influenced by the compressibility of the system and the confinement of the film. The PMMA swelling isotherms are insensitive to changing the substrate from silicon to gallium arsenide.