The positron annihilation lifetime (PAL) technique has been adopted for the study of gas permeability and the penetrant-induced plasticization of polymers. The ortho-positronium (o-Ps) lifetime parameters in fluorinated polyimides were discussed in comparison with the data from other characterization techniques for the local structure and dynamics of polymers such as solid-state (13)C NMR, quasielastic neutron scattering(QENS), X-ray reflectivity measurements and molecular dynamics (MD) simulation. In a highly plasticized state due to the sorption of propylene, the permeability and local mobility increased with the propylene pressure, and this behavior was clearly attributed to the change in the free-volume holes that could be probed by o-Ps. These results suggest that o-Ps is a powerful probe for the permeation mechanism and local dynamics in highly plasticized state of polymers.