Grain growth and the anatase-to-rutile phase transition in nanostructured titania have been investigated by electron microscopy, X-ray diffraction, and differential thermal analysis. For pure TiO2, thermal treatment resulted in complete phase transition and considerable grain coarsening, because of the grain boundary nucleation mechanism for the rutile phase. The effect of doping of the TiO2 nanophase has also been addressed. Phase transition turned out to be strongly affected by the presence of Ta and Nb because nucleation of rutile was favored at the surface of anatase grains. Doping allowed the maintenance of the ultrafine anatase phase even at high annealing temperature. Thermal analysis highlighted that oxygen mobility in the titania lattice is affected by doping and related to the inhibition of grain coalescence and phase transition.