In situ cathodoluminescence (CL) measurements have been carried out using a scanning electron microscope on 9 mol% MgO doped ZrO2 (Mg-PSZ). Light intensity measurements across the fracture surface of a piece fractured in three point bending show a position dependence of the amount of transformation observed, with a maximum occurring in the volume of sample loaded in tension prior to failure. A consistent transformation zone approximately 8 mu m wide was seen on either side of a scratch on a polished surface of Mg-PSZ.