Infrared spectra of CO2 sorbed in rubbery and glassy polymers were measured to examine the relationships between the spectroscopic data and physical properties of the polymeric membranes. The "V-shape" tendency in the plot of W1 [ i.e., half-width of CO2 peak sorbed in the membranes] vs glass-transition temperature (T(g)) is observed, and has exactly the same tendency that is widely known from the plot of log D (diffusion coefficient) vs T(g). It is suggested that the membranes having a wider W1 give a faster diffusion coefficient, since W1 is inversely related to the moment of inertia of CO2 in the membranes. Two distinct peaks of CO2 were not observed in the infrared spectra of CO2 sorbed in the glassy polymers. This suggests that the states Of CO2 in the Henry mode and Langmuir mode in the glassy polymers are similar in the spectroscopic measurements.