Infra-red spectra of CO adsorbed on NaCl, NaBr and NaI films were recorded at –196°C and for different surface coverages. The high surface area films were prepared by a method described previously.1 The absorption bands were shifted, with respect to the gas phase, towards higher energies. For NaCl as adsorbent, the heat of adsorption was calculated for various adsorption sites and orientations of the adsorbate, and was found in agreement with experiment for molecules adsorbed perpendicularly on the surface Na+ ions. A high contribution from the quadrupole field-gradient interaction was found. Spectral shifts were calculated, using the perturbation method. The perturbing potential was taken as a sum of four attractive and repulsion potentials. Good agreement with the experimental value of Δν was found.