Submicrometer elongated HfO2 particles of various axial ratios and narrow size distribution were obtained by forced hydrolysis at 100-degrees-C of HfCl4 solutions. The effects of experimental conditions (hafnium salt concentration, acidity, aging time, and the presence of the sulfate anion) oil the characteristics of the precipitated solids were systematically investigated. The powders thus prepared consisted of polycrystalline monoclinic hafnia particles generated by ordered aggregation of much smaller subunits. The change in properties of these dispersions on calcination at elevated temperatures is also reported. (C) 1994 Academic Press, Inc.