The crystallization and coarsening of pure and metal-doped (M: Ag, Fe, Co) TiO2 amorphous specimens were studied during in-situ synchrotron radiation diffraction experiments. The temperature and time dependence of the average grain-size of TiO2 anatase and rutile constituent phases was determined. Excess vacancies, secondary phases or solute elements cause the pinning of the grain-boundary motion, control the onset of the anatase-rutile transition and control the thermal stability of ultrafine microstructures in nanocrystalline TiO2 ceramics.