Abstract Amorphous poly( dl-lactic acid) (P dl LA) and poly(lactic acid- co-glycolic acid) (PLGA) polymers have been used to fabricate porous scaffolds for tissue engineering applications via a supercritical foaming technique. The chemical composition of the polymers and the morphology (pore size, porosity and interconnectivity) of the scaffolds are crucial because they influence cell filtration, migration, nutrient exchange, degradation and drug release rate. To control the morphology of supercritical foamed scaffolds, it is essential to study the interactions of polymers with CO 2 and the consequent solubility of CO 2 in the polymers, as well as the viscosity of the plasticized polymers. In this paper, we are showing for the first time that well known and useful biodegradable polymers can be plasticized easily using high pressure CO 2 and that we can monitor this process easily via a high pressure attenuated total reflection Fourier transform infrared (ATR-IR) and rheology. High pressure ATR-IR has been developed to investigate the interactions of CO 2 with P dl LA and PLGA polymers with the glycolic acid (GA) content in the copolymers as 15, 25, 35 and 50% respectively. Shifts and intensity changes of IR absorption bands of the polymers in the carbonyl region (∼1750 cm −1) are indicative of the interaction on a qualitative level. A high pressure parallel plate rheometer has also been developed for the shear viscosity measurements of the CO 2-plastisized polymers at a temperature below their glass transition temperatures. The results demonstrate that the viscosities of the CO 2-plasticized polymers at 35 °C and 100 bar were comparable to the values for the polymer melts at 140 °C, demonstrating a significant process advantage through use of scCO 2. The data from the high pressure rheology and high pressure ATR-IR, combined with the sorption and swelling studies reported previously, demonstrate that the interaction and the solubility of CO 2 in PLGA copolymers is related to the glycolic acid content. As the glycolic acid ratio increases the interaction and consequent solubility of CO 2 decreases. The potential applications of this study are very broad, from tissue engineering and drug delivery to much broader applications with other polymers in areas that may range from composites and polymer synthesis through to injection moulding.