Tetragonal ZrO2 nanocrystallites-with or without yttria (3 mol %) doping-have been synthesized via a precipitation process in which the hydrous oxide precipitate reacts with hexamethyldisilazane (HMDS) vapor before calcination. The nanocrystallites are formed and retain a tetragonal structure for hours after calcination at temperatures of 300 degrees -1100 degreesC. The enhanced structural metastability has been attributed to the combined effect of suppressed grain growth and reduced surface energy that results from the HMDS treatment.