Abstract: The invention relates to the production of alumina whiskers from alumina-bearing material by preparing an intimate mixture of such material and carbon and then passing a stream of gaseous aluminium trihalide and an inert carrier gas thereover in a first zone at an elevated temperature, usually 1400.degree.C to 1800.degree.C, at which reaction occurs to form aluminium monohalide and carbon monoxide. The gaseous stream comprising the two latter materials is then passed to a second zone maintained at a less elevated temperature than the first zone, usually 1400.degree.-1000.degree.C, at which temperature the reaction is reversed to produce a gaseous stream carrying alumina and carbon. Under conditions of low supersaturation the alumina deposits as single crystal alumina whiskers from which any carbon can be removed by an oxygen-containing gas at temperatures of 800.degree.-1000.degree.C.