Glass transition in the system poly(methyl methacrylate)/compressed gas was studied as a function of the gas pressure p using a high-pressure Tian-Calvet heat flow calorimeter. Measurements were made on PMMA-CH4,;-C2H4, and;-CO2 at pressures to 200 atm. All three gases plasticize the polymer leading to depression of the glass transition temperature T-g. Trends in the T-g depression were the same as those reported for the solubility of these gases in PMMA; the higher the solubility the larger the depression in T-g. CO2 was found to be the most effective plasticizer producing a depression of about 40 degrees C at a pressure of about 37 atm. In the low-pressure limit, the pressure coefficient of the glass transition temperature (dT(g)/dp) was found to be about -0.2 degrees C atm(-1) for PMMA-CH4, the same as that observed for polystyrene-CH4. For PMMA-C2H4, the pressure coefficient was -0.7 degrees C atm(-1), which is lower than the value of -0.9 degrees C atm(-1) observed for PS-C2H4. The pressure coefficient for PMMA-CO2 was found to be about -1.2 degrees C atm(-1), which is larger than the value of -0.9 degrees C atm(-1) observed for PS-CO2.