Steady-state permeation measurements are reported for carbon dioxide (CO2) through homogeneous polystyrene (PS)/poly (vinylmethylether) (PVME) blends. Permeability and time-lag diffusion coefficients were measured at 20 degrees C over the whole range of compositions and for CO2 upstream pressure up to 16 atm. For 80 and 70% PS glassy blends, we observe a dependence of transport properties on upstream pressures which departs from the dual mode model. This departure is explained by a plasticization due to sorbed CO2; this phenomenon is clearly confirmed by results on rubbery blends for which both permeability and time-lag diffusion coefficient increase with upstream pressure. For low upstream pressure, both permeation and diffusion exhibit a minimum in the region of the 65% PS composition which proves that the permeation is mainly governed by diffusion rather than solubility. The observed minimum conveys the non-ideal character of the PS/PVME system whose mixing leads to a volume contraction.