A comparative investigation of the phase stability at high temperatures of nanocrystalline Al2O3 and carbon-coated Al2O3@C systems was performed using a set of physicochemical and spectroscopic methods. The obtained data demonstrate that the carbon coating hinders the sintering of the δ-Al2O3 phase and its transformation to the α-Al2O3 phase at 1250 °C. Without the carbon coating, the δ-Al2O3 sinters and becomes completely converted to corundum at noticeably lower temperatures. The stabilization of the nanosized oxide particles in the Al2O3@C system was shown to be the decisive factor preventing their transformation to the α-Al2O3 phase. The thermal stability of the Al2O3@C samples calcined within a range of 1180-1250 °C in an argon atmosphere followed by the calcination in air to remove the carbon coating was found to exceed that of pure δ-Al2O3. Such samples are characterized by the presence of carbon-alumina interfaces, when carbon is encapsulated in small amounts at the places of contact between the oxide nanoparticles. Such interfaces hinder the sintering of alumina nanoparticles. It is important that the active sites present on the surface of the oxide core in Al2O3@C samples calcined in air are similar to those known for pure alumina. The high concentration of such sites after thermal treatment at elevated temperatures makes this class of materials promising for use as catalysts or catalyst supports capable of operating at high temperatures.