We present a method for the direct measurement of the glass transition temperature of compressed gas-polymer systems. The technique utilizes a Setaram C80D microcalorimeter equipped with high-pressure cells. Pressurizing the cells and running in scanning mode allows direct determination of the glass transition temperature. To validate the method, T-g measurements of the CO2-poly(methyl methacrylate) system as a function of gas phase pressure were made. The results compare favorably with literature values. However, the effects of foaming appear to interfere with T-g measurement at the highest gas pressures. The CO2-induced T-g depression of a series of polycarbonates was also measured. The magnitude of the T-g depression increases with decreasing glass transition temperature, reflecting an increase in intrinsic chain mobility, as evidenced by the glass transition temperature. The data correlate well with the Chow model.