Thin layers of caesium, rubidium and thallium halides have been evaporated on to amorphous bases at low temperatures, and examined by electron diffraction. All the caesium halides examined, as well as thallium chloride and thallium bromide, showed both the rocksalt structure as well as the normal caesium chloride structure. In general the abnormal structure gave the weaker pattern which decreased in intensity as the layer was heated and disappeared before room temperature was reached. Thallium iodide showed a caesium chloride structure at low temperatures changing to the normal orthorhombic form only in the neighbourhood of room temperature. The rubidium salts showed a rather persistent broad ring pattern at low temperatures in addition to the normal rocksalt rings. The diffuse ring pattern is consistent with a caesium chloride structure for the rubidium halides.