We report here original in situ X-ray reflectivity (XRR.) studies of thin films isothermally exposed to CO2 as a function of CO2 pressure starting from ambient pressure up to its supercritical state. Swelling of polystyrene confined films is investigated by this technique. Cycling through the supercritical state of CO2, the PS films show an irreversible behavior that is analyzed in terms of CO2 uptake that depends on P-g(h) (the pressure at which the film crosses thickness dependent T-g(h)). We evidence that this pressure is indeed thickness dependent and that CO2 can be up taken in the film in a condensed state as soon as P > P-g(h). We also evidence that the content of CO2 stored inside the film after depressurization can be determined by a careful analysis of the electron density of the film.