Abstract: A significant problem in making monocrystalline ternary semiconductor compound material, for example cadmium mercury telluride, is to produce useful quantities of the material which have electrical properties within a narrow band of values, and this depends on the material having a composition within a narrow range. Monocrystalline cadmium mercury telluride may be made by preparing a melt of the material, quenching this melt so as to produce a polycrystalline ingot 16, sealing the ingot 16 in an ampoule 13, and then forming a molten zone 20 at one end of the ingot 16 and passing this molten zone through the ingot 16 so as to form monocrystalline cadmium mercury telluride 21. The length of the molten zone 20 is from 25 to 40% of the length of the ingot 16.