A stepwise temperature- and pressure-scanning thermal analysis method was developed to measure glass-transition temperature T-g in the two-phase polymer-gas systems as a function of gas pressure p, and was used to confirm recent theoretical predictions that certain polymer-gas systems exhibit retrograde vitrification, that is, they undergo rubber-to-glass transition on heating. A complete T-g-p profile delineating the glass-rubber phase envelope was established for the PMMA-CO2 system. The retrograde vitrification behavior observed, where at certain gas pressures the polymer exists in the rubbery state at low and high temperatures and in the glassy state at intermediate temperatures, was similar to that reported previously based on the creep-compliance measurements. The existence of the rubbery state at low temperatures was used to generate foams by saturating the polymer with CO2 at 34 atm and at temperatures in the range -0.2 to 24 degrees C followed by foaming at temperatures in the range 24 to 90 degrees C. Foams with very fine cell structure never reported before could be prepared by this technique. For example, PMMA foams with average cell size of 0.35 mu m and cell density of 4.4 x 10(13) cells/g were prepared by processing the low temperature rubbery phase.