A time-dependent phase stability diagram has been developed describing the structural evolution of TiO2 precipitates in Ti-doped sapphire (alpha-Al2O3). A high-pressure TiO2 phase with the alpha-PbO2 structure (alpha-TiO2) forms as a coherent precipitate in Ti-doped sapphire annealed at 1200 degrees-1300 degrees C; extended annealing times at these temperatures result in twinned particles, the twinning minimizing the precipitate-matrix misfit strain energy. Twinned alpha-TiO2 eventually transforms to the rutile structure, coincident with the loss of coherency with the sapphire matrix. Rutile nucleates directly in sapphire annealed at temperatures at 1350 degrees C and above. Rutile is present mainly as semicoherent and twinned precipitates, the twinning occurring relatively early in the precipitation sequence.