The infrared spectra of crystalline HCl, HBr, and HI have been studied at —205°C. The fundamental vibrations were observed in HCl at 2704 and 2746 cm—1, in HBr at 2404 and 2438 cm—1, and in HI at 2120 cm—1. In addition, low-frequency bands associated with torsional lattice vibrations were observed directly and in combination with the fundamental vibrations of the molecules. It is concluded that in the low-temperature crystalline phases both HCl and HBr form zigzag hydrogen-bonded chains, the angle between adjacent molecules being about 107° in HCl and 97° in HBr. The crystalline potential function is investigated for all three molecules.