Absorption spectra of crystalline films of hydrogen and deuterium chloride and bromide at 77°khave been measured in the region 20–400 cm-1using a Michelson interferometer. The absorptions are assigned as lattice vibrations, and translational and torsional modes are distinguished. It is shown that the observed spectra are consistent with a crystalline structure of four molecules per unit cell in a planar array and approximate values of the principal intermolecular force constants are derived.