The thermal behavior of polystyrene (PS) plasticized with compressed carbon dioxide (CO2) was studied using differential scanning calorimetry with a high-pressure stainless steel pan in a sealed system. The technique proved to be a simple and convenient way to study the thermal behavior of a polymer plasticized with compressed CO2 at pressures up to 100 atm, which covers both the gas and supercritical states. A sharp fall in the decrease rate of the glass transition temperature (T-g) under conditions near the critical point of compressed CO2 was firstly observed, which corresponded with the solubility Of CO2 in PS. Since the system is scaled, which results in a stable pressure at a certain temperature, it is more suitable to study the effect of annealing. An endotherm was detected after the PS was annealed at a temperature below its T. under compressed CO2. The enthalpy of this endotherm increased linearly with increasing logarithm of annealing time under a certain pressure. The endotherm was affected by two thermodynamic equilibrations at a temperature below its T.: (i) enthalpy relaxation of the PS; and (ii) the absorption/desorption of CO2. POLYM. ENG. SCI, 49:1800-1805, 2009. (C) 2009 Society of Plastics Engineers