Interactions of polymers with compressed supercritical CO2 has been studied by using pressure-controlled scanning calorimetry (PCSC). Global cubic thermal expansion coefficients (alpha(pol-g-int)) for medium density polyethylene (MDPE) and poly(vinylidene fluoride) (PVDF) saturated with supercritical CO2 have been determined at 352.4 K over the pressure range from 0.1 MPa to 100 MPa. In both cases, the isotherms of global alpha(pol-g-int) exhibit minima near 20 MPa. At pressures below the minimum, alpha(pol-g-int) for the PVDF-CO2 system are higher than for the MDPE-CO2 system, while at pressures above the minimum the opposite was observed. This proves that incorporation Of CO2 in PVDF is stronger than in MDPE. The appearance of the minimum is attributed to the action of compressed CO2 molecules, which at higher pressures are forced to enter deep inside the interstitial or other voids in the polymer and cause their mechanical distension, which must be associated with an endothermic effect. The measurements have been performed on polymers used for fabrication of pipelines. (c) 2005 Wiley Periodicals, Inc.