Phase relations in the system MgSiO3-Al2O3 have been determined for pressures from 15 to 52 GPa and at a temperature of 2000 K from in situ synchrotron X-ray diffraction measurements obtained using sintered diamond anvils in a multi-anvil apparatus. A wide two-phase region of garnet and corundum is found to exist up to 27 GPa; at higher pressures, a phase assemblage of aluminous bridgmanite and corundum is stabilized. The solubility of Al2O3 in bridgmanite and that of MgSiO3 in corundum are highly dependent on pressure; the Al2O3 content in bridgmanite increases from 12 mol% at 27 GPa to 29 mol% at 52 GPa, forming bridgmanite with the pyrope composition (25 mol% Al2O3) at about 45 GPa. In contrast, the MgSiO3 content in corundum decreases from 24 mol% at 27 GPa to 9 mol% at 52 GPa. The minimum pressure where the pyropic bridgmanite is formed is significantly higher than the result of the earlier study (similar to 37 GPa) on the pyrope composition. These results suggest the Al2O3 content in bridgmanite and the MgSiO3 content in corundum are good pressure indicators in the mixed-phase region at pressures above 30 GPa, where virtually no pressure references are available for quench experiments. The new phase diagram in the system MgSiO3-Al2O3 confirms that bridgmanite is the dominant host mineral for Al2O3 in the pyrolitic or peridotitic compositions under the pressure and temperature conditions of the Earth's lower mantle. (C) 2016 Elsevier B.V. All rights reserved.