The sorption and transport properties of CO2 in miscible PS/PVME blends at 20 degrees C are reported as a function of pressure from 1 to 15 atm. The complex shape of isotherms for glassy blends and the concentration-dependent diffusion coefficient for rubbery blends reveal a plasticization by sorbed CO2. The significant depression in T-g has to be taken into account in the analysis of the sorption data. Diffusion coefficient for CO2 passes through a minimum when plotted against the blend composition. Such a behavior can be quantitatively related to the negative volume mixing of the PS/PVME system in the framework of the theories based on unoccupied volume.