An alumina nanocomposite containing 5.0 vol% SiC, which exhibits a fracture toughness of more than three times that of conventional alumina ceramics, was fabricated by hot pressing a powder mixture consisting of submicrometre alumina and silicon carbide powder particles at temperatures in the range 1600-1700 degrees C. SiC particles, 20-30 nm in size, were observed to occur primarily at the intragranular positions in the alumina matrix. The nanocomposite was then thermally aged in air for various periods at 1400 degrees C. The thermal treatment led to the formation of a reacted surface scale, the thickness of which increases with increasing ageing time at the ageing temperature. The microstructure of the reacted surface scale was studied using XRD, SEM and TEM equipped with an EDX analysis facility. At the thermal ageing temperature, SiC particles within the surface scale are oxidized to form silica, which subsequently reacts with the alumina matrix. The oxidation of the SIC particles and the subsequent reaction with the alumina matrix, together with the grain growth of various phases, resulted in the formation of a porous microstructure, which consists of alumina grains, mullites of differing composition, and amorphous silica and aluminosilicate pockets. Two types of mullite phase, which contain a high and a low level of silica, respectively, were identified in the surface scales. The high silica-containing mullite phases, the composition of which is close to that of stoichiometric mullite, occur as large, irregularly shaped matrix grains. The low silica-containing mullite phases (15-20 wt% SiO2), which exhibit a rounded morphology, are observed to occur as second-phase particles entrapped within the high-silica containing mullite grains. The nanocomposite structure in the bulk region remains almost intact when compared with that of the unaged nanocomposite. The only noticeable difference is that the alumina matrix in the thermally aged nanocomposite exhibits a slightly larger grain size than that of the unaged nanocomposite.