The thermal and chemical stabilities of group-III sesquioxides (Al2O3, Ga2O3, and In2O3) were comparatively investigated at an atmospheric pressure at heat treatment temperatures ranging from 250 to 1450 degrees C in a flow of either N-2 or H-2. In a flow of N-2, the thermal decomposition of alpha-Al2O3 was not observed at the temperatures investigated, while the decompositions of beta-Ga2O3 and c-In2O3 occurred above 1150 and 1000 degrees C, respectively, with no generation of group-III metal droplets on the surfaces. In contrast, the chemical reactions of alpha-Al2O3, beta-Ga2O3, and c-In2O3 began at low temperatures of 1150, 550, and 300 degrees C in a flow of H-2. Thus, the presence of H-2 in the gas flow significantly promotes the decomposition of group-III sesquioxides. The order of thermal and chemical stabilities (alpha-Al2O3 amp;gt;amp;gt; beta-Ga2O3 amp;gt; c-In2O3) obtained experimentally was verified by thermodynamic analysis, which also clarified dominant decomposition reactions of group-III sesquioxides. (C) 2016 The Japan Society of Applied Physics