Microcalorimetry and vibrational spectroscopy have jointly been used to study the interaction at room temperature between CO2 and Na-ZSM-5. This interaction consists of a reversible, two-step adsorption on sites approximately all equal and noninteracting, which constitute an ideal ensemble in the thermodynamic sense. From the volumetric and calorimetric adsorption isotherms, standard changes in enthalpy and Gibbs free energy due to adsorption were calculated for both the 1:1 and 2:1 adducts formed by CO2 and Nai ions, and from these data, the standard changes in entropy were determined. All vibrational modes of adsorbed CO2 have been measured, including the usually elusive intermolecular ones, thus allowing the entropy of the adsorbed phase to be calculated by means of statistical mechanics. The vibrational modes of the carbon dioxide molecule are only slightly perturbed from the corresponding values for the gas phase. The most prominent vs mode is at 2356 cm(-1) in the 1:1 adduct and at 2352 cm(-1) in the 2:1 adduct, as compared to 2349 cm(-1) in the gas phase. The yl and Ya modes are at 1382 and 656 cm(-1) for both adducts (1388 and 667 cm(-1), respectively, in the gas phase). Comparison between volumetric and optical adsorption isotherms has allowed the estimate of the related molar adsorption coefficients for all vibrational modes of the two adducts, with the exception of the corresponding value for the nu(3) mode in the 2:1 adduct, because the related band is exceedingly intense.