Both chloride-free (amorphous, group A) and chloride-containing (crystalline, group B) ZrO2-8.1 mol % MgO powders produced by means of gelatinized coprecipitation have been calcined at 450 to 1450° C and characterized by differential thermal analysis/thermal gravimetric analysis, X-ray diffraction, electron microscopy, and infrared spectroscopy. For both groups A and B, the powder density, chloride content, crystallite size, crystallization temperature, and the initial temperature for metastable tetragonal-to-monoclinic (Tm → M) transformation decrease with increasing concentration of NH4OH with which samples were prepared. High-temperature tetragonal-to-cubic transformation of group B also revealed the same relationship, but the inverse for group A. An explanation based on the nature of the Zr-O bond and internal strain have been proposed for both transitions. In addition, it is also found that silica can inhibit both transitions and raise the critical crystallite size for Tm → M transitions, but water vapour and chloride have the inverse effect on the Tm → M transition.