A rapid in situ diffraction technique, synchrotron radiation–energy dispersive diffraction, has been used to obtain direct information on the kinetics of the tetragonal-to-monoclinic transformation temperatures for zirconia synthesized by heating the hydroxide up to temperatures between 900 and 1300 °C. In all cases the monoclinic phase appears only during the cooling stage. The effects of chemical (hydroxide preparation–pH) and heat treatments (top dwell temperature and dwell time) have been investigated. The transformation temperature increases with both top temperature and dwell time. For the lesser heat treatments (generally temperatures <1000 °C) the high-pH material displays the higher transformation temperatures, whereas with greater heat treatments (generally temperatures >1000) the low-pH material displays the higher transformation temperatures. These data do now give reliable indications on how to design a required metastability into the final room temperature product.