AbstractThe vibrational frequency shifts and splittings of polyatomic molecules upon adsorption are discussed in the harmonic approximation. Equations relating the shifts and splittings of the normal modes of the adsorbed molecule to its position and orientation on the surface are derived on the basis of interactions built up by summation over all atoms of the vibrating molecule. As an example, the shifts and splittings of the vibrational energy levels of CO2 on the {100} face of NaCl are calculated. The calculated values approach very well those observed. The infrared transmission spectra of CO2 adsorbed on NaCl films were recorded at ‐ 160°C. Upon adsorption new bands were observed due to the symmetric v1 vibration and one component of the 2v2 transition, which are in Fermi resonance and infrared inactive in the free molecule, and a well‐resolved splitting of the v2 transition probably caused by the removal of its degeneracy. As are all other frequencies, those of the v3 transition of both 12CO2 and 13CO2 are shifted to lower values.