In situ interferometric observation of the process of CO2 dissolving into PMMA (polymethyl methacrylate) was carried out at high pressure and room temperature, in view of the interest to polymer foam technology. By analysing interferograms, the phase shift of a laser beam transmitted though the sample (the phase is related to the dissolved gas content and polymer thickness) was obtained as a function of time and pressure. At constant pressure, the phase shift initially increased over time and reached a maximum value, and then decreased asymptotically to a constant value as equilibrium was approached. In addition, the equilibrium value of the phase shift increased with increase in pressure for pressures lower than the glass transition pressure Pg, and decreased at pressures higher than Pg. Thus, the optical properties exhibited during the process of the gas dissolving were drastically different for the glassy state and the rubbery state of the polymer.