Polymorphism of the methylchloromethane compounds, (CH3)nCCI4-n, where n varies from 0 to 4, has been studied using low-temperature X-ray diffraction techniques. Single crystals of both high and low-temperature phases were grown and crystal data were obtained for the various phases. The crystal structure of the phase of t-butyl chloride which is stable between −54 and −90° was determined. On the basis of the present investigation, the conclusion is reached that the high-temperature modifications are dynamically disordered, in agreement with the results of n.m.r. and dielectric studies.