The glass transition temperatures of polycarbonate (PC) were measured by means of high pressure DSC in He, N-2, and CO2 atmospheres. It was found that the glass transition temperature (T-g) of PC was independent in He with pressure up to 70 atm, whereas the T-g of PC shifted by 6 degrees towards the lower temperature end in Na atmosphere when the pressure was 70 atm. The depression in T-g by high pressure CO2 was found to be even more remarkable due to a stronger plasticization effect. It was also found that the glass transition for PC/CO2 system exhibited an endothermic peak. It was believed that the endothermic behaviour resulted from the excessive desorption of the absorbed gas at T-g due to the increase in chain mobility. This conclusion was further confirmed by examining the glass transition behaviour of poly(vinyl benzoate) in CO2 at 2 atm.