Measurements have been made of the solubilities and effective diffusion coefficients at 25 degrees C for carbon dioxide in random and uniaxially oriented samples of poly(ethylene terephthalate) and poly(ethylene naphthalate) (PET and PEN). Most measurements were made using a double-sided experiment in which the penetrant entered both sides of the sample simultaneously and the concentration was monitored as a function of time by means of infrared spectroscopy. Pressures of carbon dioxide from 0.125 to 5 atmospheres were employed. Finite difference modeling was used to determine concentration-dependent diffusion coefficients from the effective diffusion coefficients measured directly and to compare the results with the dual-mode and gas-polymer matrix models. The results could be fitted slightly better by the dual-mode model than by the gas-polymer matrix model, although the latter also provided a reasonable fit to the data. There was, however, no observed change in the spectrum of the absorbed carbon dioxide with concentration, as would be expected if it existed in two distinct states within the polymer, as required by the dual-mode model. No definite conclusion can thus be drawn about the applicability of the two models to the systems studied.