The rate of the transformation from the metastable tetragonal to the monoclinic phase of ZrO2 was measured in air from 850 degrees C to 1000 degrees C by neutron diffraction. This rate was found to be temperature dependent, and its measured values were considerably lower then those reported previously. The kinetics of this phase transformation is discussed in terms of a modified ＇crystallite growth-martensitic transformation model＇ that includes the distribution of crystallite sizes.