The CO2 sorption isotherms of samples of polystyrene/polycarbonate blends with different compositions which were prepared by casting from 1,4-dioxane solutions were obtained as a function of composition of the polymer blend at 25 degrees C using a Cahn-sorption apparatus. The characterization of the polymer blend system was obtained from the point of thermal behaviour such as differential scanning calorimetry, X-ray diffraction, and C-13 CP/MAS n.m.r. From the characterization of the polymer blend, it was confirmed that there was a small amount of crystallinity of polycarbonate. Basically the polymer blend system studied here is heterogeneous (i.e. phase-separated) but it was found that a small amount of polystyrene is partly miscible with polycarbonate and this probably disrupts the crystallization of polycarbonate especially at the higher polystyrene content. CO2 sorption isotherms of the blend system including two homopolymers appeared to obey the dual mode sorption isotherm, which is a characteristic of the glassy state. The change of the CO2 sorption amount for the polymer blend was mainly explained by the amount of the crystallite of polycarbonate, indicating that the effect of the partly miscible region on the CO2 sorption behaviour is very small.