A Tian-Calvet type heat-flow calorimeter has been modified for use at pressures up to 400 bar, and has been used for studying the glass transitions in the systems polystyrene (PS)-methane (CH4), -ethylene (C2H4), and -carbon dioxide (CO2). All three gases plasticize PS leading to lowering of the glass transition temperature Tg. The low-solubility gas CH4 induces somewhat weak plasticization; the Tg decreases gradually, with a dTg/dP value of about — 0.2 K bar-1, for pressures up to 150 bar after which the Tg remains almost constant for pressures up to 350 bar. At higher gas pressures, CH4 acts more like a hydrostatic-pressure generator than as a plasticizer such that the hydrostatic effect, which tends to raise the Tg, cancels out any additional plasticization effect. The moderately soluble gases, C2H4 and CO2, show rather strong plasticization effects with a dTg/dP value of about — 0.9 K bar-1 for both gases. The highest pressures at which the glass transition in the two-phase polymer-gas systems PS-C2H4 and PS-CO2 could be measured were 88.0 and 60. 0 bar, respectively. For the PS-CO2 system where gas solubility data are available, the measured Tg's agree very well with the values calculated from the model proposed by Chow .