A study of the solid-state oxidation of evaporated aluminum (Al) films on c-plane sapphire was undertaken. The thickness of the films ranged from 100 to 1000 nm, and the samples were subjected to isothermal annealing in air, at temperatures from 350 degrees to 650 degrees C. For heat-treated samples, the removal of remnant metallic Al using a 45% KOH etch revealed the formation of novel, sub-surface oxidation features at the Al-sapphire interface. These features were associated with the hillocking of the Al film, and consisted of hollow, truncated pyramidal structures. Low ridge-like walls were observed around the periphery of the bases of these structures. The ridges exhibited a crystallographic relationship with the sapphire substrate; a corresponding relationship with the overlying Al film, due to its highly textured nature, was inferred. Transmission electron microscopy of sections through the ridges revealed that they consisted of alumina. It is believed that during heat treatment, voids develop in the Al in order to relieve the strain energy due to compressive stresses, and that faceting of the voids subsequently takes place due to surface energy considerations. The walls of the pyramidal structures are thought to have formed by the oxidation of the surfaces of the mentioned facetted voids.