The description of structural relations between bixbyite- and corundum-type structures is of particular interest because of the common occurrence of both structures. One of the representative examples of the bixbyite to corundum transition is the high-pressure high-temperature synthesis of the corundum-type indium oxide. The wet chemistry synthesis and stabilisation of the corundum-type In(2)O(3) under ambient pressure conditions calls for a re-interpretation of the In--O phase diagram as well as for the clarification of the phase transitions in In(2)O(3). One of the questions to be clarified is the stability of the corundum-type In(2)O(3). In the present work we studied the stability of the corundum-type In(2)O(3) both theoretically (by density-functional calculations) and experimentally. The synthesis of the corundum-type In(2)O(3) was performed by the modified non-alkoxide sol-gel method based on the ammonia-induced hydrolysis of indium nitrate in methanol. The corundum-type In(2)O(3) was subjected to thermal analysis (STA) as well as to structural studies, that is, it was examined using X-ray powder diffraction (XRPD) including in situ XRPD characterisation upon thermal treatment. For the first time we have undoubtedly demonstrated, both theoretically and experimentally, the metastability of the corundum-type In(2)O(3) polymorph. The In(2)O(3) polymorph appears to be metastable throughout the entire enthalpy-pressure phase diagram. Upon heating, corundum-type In(2)O(3) transforms irreversibly into cubic bixbyite-type In(2)O(3) as shown by STA as well as in situ heating XRPD experiments. Computations indicate the existence of another high-pressure modification of In(2)O(3) with orthorhombic structure, iso-typic to Rh(2)O(3)-II. We predict this new phase to form at pressures exceeding 15 GPa from both the cubic bixbyite-type and the corundum-type modification of In(2)O(3).