In general, pressurized CO2 is soluble in polymers (to it concentration determined by the pressure) and affects the polymer relaxation behavior. Focusing on segmental relaxation, this study conducted dielectric measurements for racemie DL-poly(lactic acid) (PLA) as well as polystyrene (PS) under carbon dioxide (CO2) pressurized up to P-CO2 = 3.5 MPa. The dielectrically detected segmental dynamics of PLA and PS wits found to be accelerated oil the increases office CO2 pressure P-CO2 that resulted in an increase of the CO2 concentration (detected through a decrease of the dielectric intensity). In this sense, the pressurized CO2 behaved as an ordinary solvent. However, the dielectrically detected segmental relaxation mode distribution was quite insensitive to P-CO2 which was in strong contrast to the mode broadening induced by ordinary solvents such as di-n-butyl phthalate (DBP) (the broadening was confirmed also in this study for PLA plasticized with DBP). The mode broadening due to ordinary solvents is attributable to spatial fluctuation of the solvent concentration that results in a distribution of it local segment concentration, thereby giving a distribution of the segmental relaxation time tau(s). Thus, the lack of this broadening seen for file PLA/CO2 and PS/CO2 Systems Strongly Suggested lack of an effect of the CO2 concentration fluctuation on the tau(s) distribution. This unique feature of CO2 was attributable to the motion Of CO2 molecules being in much faster than the segmental motion of polymers: The fluctuation of CO2 concentration was possibly smeared through the CO2 motion before the dominant part of the segmental relaxation occurred, thereby giving no significant broadening of the tau(s) distribution.