The phase diagram of the Lu2O3-Al2O3 system has been defined showing all three compounds existing in the system, i.e. with molar ratios 2:1, 1:1 and 3:5, which exhibit strongly differing stabilities. The range of stability of the perovskite phase has been estimated to lie approximately in between 1750 and 1930 degrees C. The range of solid solutions of the garnet phase and the lower border of stability of the monoclinic phase are also shown in the diagram. (c) 2006 Elsevier B.V. All rights reserved.