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In general, pressurized CO 2 is soluble in polymers (to a concentration determined by the pressure) and affects the polymer relaxation behavior. Focusing on segmental relaxation, this study conducted dielectric measurements for racemic DL-poly(lactic acid) (PLA) as well as polystyrene (PS) under carbon dioxide (CO 2 ) pressurized up to P CO 2 =3.5 MPa. The dielectrically detected segmental dynamics of PLA and PS was found to be accelerated on the increases of the CO 2 pressure P CO 2 that resulted in an increase of the CO 2 concentration (detected through a decrease of the dielectric intensity). In this sense, the pressurized CO 2 behaved as an ordinary solvent. However, the dielectrically detected segmental relaxation mode distribution was quite insensitive to P CO 2 , 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 a local segment concentration, thereby giving a distribution of the segmental relaxation time, τ s . Thus, the lack of this broadening seen for the PLA/CO 2 and PS/CO 2 systems strongly suggested lack of an effect of the CO 2 concentration fluctuation on the τ s distribution. This unique feature of CO 2 was attributable to the motion of CO 2 molecules being much faster than the segmental motion of polymers: The fluctuation of CO 2 concentration was possibly smeared through the CO 2 motion before the dominant part of the segmental relaxation occurred, thereby giving no significant broadening of the τ s distribution.