A novel apparatus for in situ optical measurement of glass transition temperature （Tg）was developed.The apparatus consisted of a high-pressure cell equipped with quartz windows

a data acquisition system

a heating and temperature-controlling system

a pressure-controlling system

and a CO2 feeding system.The volume changes of poly-(ethylene-terephthalate) (PET) when it was swollen by CO2 were measured in real-time by the apparatus

at the temperatures of 50

60

70

80

90

100 and 120℃

and under the pressures of 0.1

5

10

15

20 MPa of CO2.Consequently

the glass transition temperatures in SC-CO2 were obtained under various conditions as indicated above.To examine the accuracy and reliability of this method

comparison of the results obtained by the apparatus under atmospheric pressure with those obtained by the standard differential scanning calorimetry (DSC) were made

and these two methods produced results within acceptable precision (1.6%).Thus the method developed in this work can be reliably used to measure the glass transition temperature of polymers in SC-CO2.