The mechanical response of liquid nitrogen-quenched 9.4 Mg-PSZ in which the orthorhombic (o) phase is the major constituent (46 wt%) was investigated using in-situ neutron diffraction during uniaxial compression. The material remains elastic below 1 GPa with a Young's modulus of similar to 242 GPa, second highest of all zirconia-based materials and highest of all zirconia-based ceramics. Beyond I GPa, the material develops small plastic strains in a time-dependent manner (i.e., by room temperature creep) although the strains were generally much smaller than the unquenched material, which contains no o phase. As for standard Mg-PSZ, the creep was accompanied by a volume change usually indicative of tetragonal to monoclinic (m) phase transformation; however, the amount of in phase apparent in the neutron diffraction patterns increased only marginally. The magnitude of the volume increase could not be accounted for by the observed increase in the in phase and hence, microcracking is believed to be responsible for most of the volume change. There is some evidence for a small amount of o to in transformation at the detection limit of the phase analysis technique.