Submicron hafnium oxide powder prepared by hydrolytic decomposition of alkoxides was studied. The particle size range of this powder was 10 to 50 Å. Emission spectrographic analysis of the powder after it was calcined at 250°C for 0.5 h indicated a purity of>99.995%. Up to 320°C, the powder showed no crystallinity by X‐ray analysis. The amorphous HfO2was isothermally aged at 5° to 10°C intervals between 200° and 500°C. X‐ray diffraction patterns indicate a sharp transition from an amorphous state to the monoclinic phase at 325°C. High‐temperature X‐ray studies and DTA suggest nucleation and growth of small crystallites at 420°C leading to conversion to monoclinic HfO2at 480°C. BET surface area measurements and TGA of the powders were also conducted. A powder which transformed at 325°C to the monoclinic phase was isothermally aged below 325°C for 1