Phase transformation by grinding has been studied in heat-treated 2.5% molar Y-TZP with a duplex tetragonal-cubic microstructure developed by exposure at 1650 degrees C for different times as well as in specimens with a large amount of monoclinic phase induced by prior aging at 300 degrees C in air. For the grinding process used, it is shown that no monoclinic phase is detected in high transformable microstructures with a relatively large tetragonal grain size. Additional experiments are discussed in which it is shown that, by grinding previously aged specimens with large fractions of monoclinic phase, the amount of this phase is strongly reduced after grinding.