The thermal shock resistance (TSR) of Mg-PSZ (a partially stabilized ZrO2) containing either tetragonal t- or monoclinic m-ZrO2 precipitates was studied. The samples containing t-ZrO2 had superior TSR, particularly in having a higher retained strength after thermal fracture had been initiated. The t-ZrO2 in the surface regions of shocked samples transformed to a new polymorph with orthorhombic (o) symmetry; the stress-induced t → o transformation may be responsible in part for the improved TSR in samples containing t-ZrO2 compared to those in which all the particles have m symmetry.