Aqueous solutions of boehmite and hydrofluoric acid (HF) were used to prepare homogeneous mixtures of alumina and aluminum fluoride. Calcination at temperatures as low as 1000 degreesC resulted in the formation of well-defined hexagonal-shaped alpha -alumina platelets. Containment of the aluminum fluoride by covering the calcination crucible promoted crystal growth presumably by a reaction of continuous evaporation-condensation of aluminum fluoride. Hexagonal-shaped platelet alpha -alumina was observed with average diameters ranging from 7 to 33 mum. Large platelets with a narrow size distribution and average diameter of over 25 mum were prepared by controlling the initial concentration of HF and the calcination time, temperature, and atmosphere.