Differential scanning calorimetry was used to measure the heat evolved as zirconia samples are heated at constant rate. The data are used to deduce the kinetics of zirconia crystallization during hydrothermal treatments at pH 2, 7, 9.5 and 13. Hydrothermal crystallization was found to proceed much more slowly in the neutral pH medium than in the acidic or alkaline medium. The analogy between the rate of the hydrothermal crystallization of ZrO2 and solubility of zirconium hydroxide was discussed. The results of phase analysis showed that metastable cubic zirconia, c-ZrO2, could be produced by hydrothermal crystallization in the presence of NaOH as a stabilizing agent. The results of X-ray diffraction showed that, even at low pH, monoclinic zirconia, m-ZrO2, was not the only product of the dissolution/precipitation mechanism of hydrothermal crystallization. It is concluded that in situ crystallization of amorphous zirconium hydroxide, if it occurs, is not a topotactic process.