Supercritical CO₂ is an environmentally responsible solvent with tunable physical properties. A diverse array of polymerizations and polymer processing techniques has been reported using CO₂. Foaming or CO₂ swelling occurs when a polymer is placed in contact with supercritical CO₂. CO₂ swelling can either be used to extract or add materials into a polymeric substance. A high-pressure isothermal calorimetric method has been developed to observe alterations in the glass transition temperature (T_g) of poly(acrylic acid) (PAA) due to the adsorption of gaseous and supercritical CO₂. PAA is a hydrophilic commodity polymer used as a thickener and a water absorbent (e.g. baby diapers). Recently, Liu and coworkers (Liu,T.,Continuous Precipitation Polymerization of Acrylic Acid in Supercritical Carbon Dioxide,2005 ) have demonstrated the ability to synthesize PAA in supercritical CO₂ via continuous precipitation polymerization. Theoretical calculations of CO₂-induced T_g depression, stemming from the amount of CO₂ adsorbed in PAA, suggest that the morphology of PAA particles depend on experimental operating conditions during precipitation polymerization and the proximity of these conditions to T_g. The goal of this project is to evaluate (i) T_g for PAA polymerized in supercritical CO₂ ,(ii) the reduction of T_g for PAA in the presence of CO₂ (20.7 to 206.8 bar), (iii) and the validity of the theoretical model for T_g depression. CO₂-induced T_g depression was investigated in custom designed high-pressure sample cells using a Setaram C80 calorimeter. PAA samples synthesized in supercritical CO₂ (206.8 bar, 50 to 90°C) have a T_g ranging from 114 -120°C, which is analogous to PAA that is produced conventionally by solution polymerization. PAA adsorption of compressed CO₂ yielded a reduction in the T_g to 100°C at 58.7 bar ± 0.6. In the presence of supercritical CO₂ the T_g reduced to 80°C at 126 bar. These reductions in T_g are consistent with theoretical calculations for T_g approximation using the Chow's equation.