Infra-red spectra of HCN adsorbed on NaCl, NaI and CsCl films were recorded at low temperatures and for different surface coverages. The films were prepared in a specially designed low-temperature cell by deposition of vapours on a NaCl window cooled to –196°C. The surface areas were in the range 200–300 m2/g. HCN adsorbed on NaCl gave a strong and broad band at 3.18 µ with a shoulder at 3.29 µ, both due to the v3 vibration. These bands are shifted with respect to the gas phase absorption by 170 and 270 cm–1 towards lower energies. There appeared a narrow strong band at 4.77 µ due to the v1 mode not present in the gas phase. Under better resolution this band splits into two absorptions about 10 cm–1 apart. The v2 bending mode appeared as two absorptions, at 12.5 µ and 13.4 µ, the latter as a well-resolved doublet with a spacing of 13 cm–1 at –78°C and 23 cm–1 at –196°C. This doublet is explained by the removal of the degeneracy of the bending mode in molecules adsorbed parallel to the surface. The 12.5 µ band probably results from molecules oriented perpendicular to the surface. On NaI films the spectrum in the high frequency region resembles that obtained with NaCl films. At low frequencies the v2 band appears very flat without any signs of splitting. HCN on CsCl shows two absorptions for the v3 mode at 3.16 µ and 3.32 µ due to adsorption of the molecule on two different sites arising from two different crystalline structures of the substrate. The v2 band does not show any structure and its maximum is at 12.05 µ. Spectral shifts of the v3 mode were calculated using the perturbation method and anharmonic oscillator wave functions. The peturbing potential was taken as a sum of dispersion, electrostatic, induction and repulsion potentials. The magnitudes of these potentials were obtained through summation of interactions with hundreds of ions in the underlying crystal adsorbent.