A pressure-temperature diagram of In2O3 was determined at pressures up to 9 GPa and temperatures between 20 and 2000 degrees C by in situ X-ray diffraction measurements. In2O3 decomposed into indium and oxygen upon heating. The decomposition temperature rose from 1250 to 1700 degrees C as the pressure increased from 2.0 to 4.0 GPa, but remained near 1700 degrees C at higher pressures. The change in the pressure dependence was attributed to a phase transition; the cubic structure transformed into a rhombohedral corundum structure at 4.0 GPa and 1600 degrees C. The phase boundary had a slightly negative slope and its extrapolation estimated a transition pressure of 12 GPa at room temperature. Rapid grain growth was observed near the phase boundary in the rhombohedral region at temperatures above 1000 degrees C, which is still sufficiently below the decomposition temperature. Single crystals of the high-pressure rhombohedral phase were recovered at ambient conditions. (C) 2007 Elsevier B.V. All rights reserved.