Thermal and spectroscopic properties of the annealed cast syndiotactic polystyrene (SPS) films were investigated and it was shown that the sample annealed at lower temperature than 170 degrees C exhibited the presence of a conformational order but lack of crystalline regularity. This result indicated the formation of mesophase by annealing cast SPS films. CO2 sorption properties of such a film were also examined and dual-mode sorption model was applied to clarify the SPS structure. The hole saturation constant for the Langmuir sorption (C-H＇) evaluated from the dual-mode sorption model reflected the fine structure of the annealed SPS (mesophase) and depended on the annealing time when annealed at a certain temperature. From these results we supposed that the samples annealed at lower temperature contained many nanopores between SPS chains of mesophase, i.e. desorption sites of solvent.