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 Na+ 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 ν3 mode is at 2356 cm-1 in the 1:1 adduct and at 2352 cm-1 in the 2:...