The crystal structures of (NH4)2TeBr6 and Ce2TeBr6 have been reinvestigated using X-ray diffraction from powders and single crystals. Three dimensional single crystal intensity data obtained photographically have been used to refine these structures, which are of the cubic K2PtCl6 type, by a full matrix least squares analysis. The Te-Br bond is found to be 2.70* in both crystals after correction for the thermal notion of the atoms. A phase transition in (NH4)2TeBr6 has been observed at about 183°K. Pros an examination of Weissenberg photographs taken at 163°K, the low temperature phase is found to be tetragonal, space group p4/mnc (D64h) with a « 7.501+/-0.005* and c =10.765*+/-0.005*. The transformation in this case is probably similar to that reported in other isomorphous crystals, e.g., K2snBr6. A review of other crystals with similar structure is included in this thesis and the theory of X-ray diffraction as applicable to the present problem is discussed briefly. / Thesis / Master of Science (MSc)