Bulk nanocrystalline TiO2 samples (100% rutile) with a relative density as high as 97% and a grain size of <20 mn have been produced via high-pressure (up to 8 GPa)/low-temperature (similar to 0.3T(m), where T-m is the melting temperature) sintering, using a toroidal-type high-pressure apparatus. Nanophase TiO2 powder with a metastable anatase structure and an initial grain size of similar to 38 nn was used as the starting material. During sintering, the anatase phase transformed to either the rutile or srilankite phase, depending on the pressure-temperature (P-T) combination. The starting temperature of the anatase-to-rutile phase transformation decreased from similar to 550 degrees C at ambient pressure to similar to 150 degrees C at 2.5 GPa, Grain growth was limited by the low sintering temperature and the multiple nucleation events in the parent phase, The grain size of the transformed rutile decreased as the sintering pressure increased, which can be explained by the combined effect of increasing the nucleation rate and decreasing the growth rate with high pressure. We have demonstrated that it is possible to produce a dense sintered compact with a grain size even smaller than that of the starting powder. The high-pressure srilankite phase was observed at P-T conditions as low as 4.75 GPa and 250 degrees C, respectively; however, unlike the anatase-to-rutile phase transformation, the rutile-to-srilankite phase-transformation temperature increased as the pressure increased, Also, in contrast to the irreversible anatase-to-rutile phase transformation, the srilankite will reversibly transform to rutile under the appropriate circumstances. This observation provides an opportunity to further refine the TiO2 grain structure by switching the sintering conditions (temperature and pressure) between the regions in which the rutile or srilankite phase are stable.