Mean square thermal displacements of the chlorine and deuterium atoms were derived both in the ordered orthorhombic and in the disordered cubic phase of deuterium chloride by a least-squares study of the neutron powder diffraction patterns recorded at four temperatures over the 4.2 to 111.5 K range. The atomic displacements were subsequently interpreted as due to the superposition of the internal stretching vibrations and of the rigid-body translational and angular vibrations of the molecules. The analysis of the results revealed substantial rigid-body vibrational components in the zero-point motion of the molecules and a rapid rise in the amplitude of the in-plane angular vibrations on approaching the transition point. It also showed a small but possibly significant lengthening of the CI—D bond as the temperature decreases. Finally, it gave a fuller understanding of the disordered cubic phase as a dynamic mixture of short-lived polymers of varying lengths and shapes.