We report a method for producing stable low-density polymer films by using supercritical carbon dioxide ( scCO ₂). Two different molecular weight polystyrene films with various thicknesses were exposed to scCO ₂ along the density fluctuation ridge in P–T phase diagram. The swollen structures could be then frozen by flash evaporation of CO ₂ without forming additional voids. X-ray reflectivity data clearly showed that exposure to scCO ₂ could be used to produce uniform low-density films of about 2R_g thick or less, where R_g is radius of polymer gyration. © 2003 American Institute of Physics.