Hydrothermal crystallization of HfO2 from aqueous suspensions of hydrous hafnia at different pH values and temperatures were monitored using differential scanning calorimetry/thermogravimetric (DSC/TG) analyses and powder X-ray diffraction (XRD). Both techniques showed that the hydrothermal crystallization proceeded much more slowly in a neutral pH medium than in an acidic or alkaline medium. However, the rate of this process appeared to be much slower according to XRD. The results of XRD analysis indicated the presence of a certain incubation time, whereas DSC/TG analysis indicated an almost immediate start of the crystallization process. The reasons for the obtained differences were discussed. The results of phase analysis of the products of hydrothermal treatment showed that, regardless of pH, hydrous hafnia crystallized into m-HfO2. Stabilization of a metastable cubic or tetragonal HfO2 occurred upon adding a sufficient amount of stabilizing Na+ ions. It was concluded that hydrothermal crystallization of HfO2 proceeds via a dissolution/precipitation mechanism in the whole pH range. (C) 2004 Elsevier B.V. All rights reserved.