Monoclinic zirconia has been synthesized hydrothermally from zirconium oxychloride in the range 20–100 MPa, 250–650°C, for run duration from 20 to 100 h and in the presence of NaOH, Na2CO3, Na2SO4, NH4F, HNO3, and H2SO4additives. Isometric, platelike, and elongated crystal morphologies were obtained depending on the additive. Both spherulitic and isolated textures were encountered with H2SO4. Growth of isometric crystals follows a general dissolution–precipitation mechanism. For H2SO4, two growth steps were identified: an early spherulitic step followed by an isolated crystals step resulting from Ostwald ripening of preexisting spherulites. The formation of spherulites is consistent with the specific properties of the sulfuric hydrothermal