This thesis examines the effects derived from the ability of high pressure carbon dioxide to soften polymers. This has potential applications in the shape forming of polymers at lower temperatures, dye impregnation and the foaming of polymers. This study was conducted in two parts: (i) mechanical measurement of polymer softening under CO2 at high pressure; and (ii) foaming behaviour of polymers containing dissolved CO2 during depressurisation. In the first study the softening of polymers as a function of applied CO2 pressure and temperature was measured using a novel mechanical 3-point bend test rig. In initial experiments the temperature was slowly ramped upwards and the nominal glass transition temperature was recorded as where the central deflection suddenly begins to increase. Significant reductions in the bending onset temperatures were observed on the application of CO2 for polycarbonate, poly(methyl-methacrylate), glycol modified poly(ethylene-terepthalate) and polystyrene, of typically 50–100°C over the range of pressures applied (24 to 120 bar).