Polymers are currently processed in many different ways involving elevated temperatures and pressures as well as additional chemicals (mostly gases), this, being typically the case in foaming process. There is then a pressing need to assign the thermophysical properties of polymers in particular states under precise thermodynamic conditions to obtain optimal performances. Thermal, mechanical and/or chemical stresses may induce modifications, possibly permanent, of the glass transition which consequently affect the characteristics of the material. Polystyrene (PS) has been submitted, in a scanning transitiometer (ST), to pressures up to 200 MPa using as hydrostatic fluid either mercury as a neutral fluid or methane as a "chemically active" fluid. Temperature modulated differential scanning calorimetry (TMDSC) has been used to study the thermal behavior, especially the glass transition, of polystyrene modified by high pressure methane, in comparison with polystyrene submitted to high pressure mercury as well as with "non-treated" (native) polystyrene.