The devitrification of polystyrene, PS, thin films, supported by oxidized silicon (SiO,/Si) wafers was examined under various conditions of CO2 pressure and temperature using in situ spectroscopic ellipsometry. PS films in the thickness range of 15 nm < h < 200 nm were studied at temperatures from 25 to 75 degreesC. The CO2 pressures, P-g at which devitrification of the PS films occurred decreased with decreasing film thickness for h less than or similar to 50 nm at each temperature, indicating the effect of excess CO2 in the free surface and substrate regions. For thicker films, P-g remained constant. The largest drop of the transition pressure in this thickness range (15 nm < h < 50 nm) occurred at 75 degreesC. The phenomenon of retrograde vitrification is shown to occur in this system, wherein with decreasing temperature isobarically a PS film would first undergo the expected rubbery-to-glassy transition and at yet lower T the film would subsequently exhibit a glassy-to-rubbery (devitrification) transition. Retrograde vitrification is characterized by a vitrification envelope below a maximum pressure (similar to60 atm) on a P versus T plot. This phenomenon is absent in bulk PS.