Opportunities exist to exploit the unique properties of carbon dioxide to process polymers for advanced applications. The diffusion of deuterated water in poly(methyl methacrylate) films swollen by carbon dioxide at 35 degrees C over the pressure range 0 to 90 bar (corresponding density range : 0.0 to 0.66 g/cm(3)) was measured using in situ FTIR spectroscopy. The mechanism and time scale of the diffusion of deuterated water in PMMA are easily controlled by manipulation of the applied CO2 density. The enhancement of mass transport rate is consistent with the degree of plasticization of the polymer clue to the absorption of carbon dioxide.