The various isothermal and photolytic decomposition stages of α-aluminum hydride have been examined by scanning electron microscopy. The pristine material consists of cuboids, 50-100 μm on a side, some of which are contact or interpenetrating twins. The interpenetrating twin axis appears to be the  body diagonal of the cuboids. The cuboids' surface, which has been chemically treated to promote thermal stability, contains faceted pits whose sides lie along close-packed directions. The pit interiors show evidence of layering. During the thermal induction period, patches containing acicular filaments, each ∼0.005 μm in diameter and ∼0.3 μm in length, of Al-metal are formed and are dispersed randomly on the internal crystal surfaces. During the acceleratory period, a large number of additional patches appear in which some of the filaments cluster together to form small clumps. The acicular filaments in all patches thicken and increase slightly in length as the reaction proceeds. Later in the acceleratory period, bubbles (0.2 to 3.5 μm in diameter) appear. The bubbles are randomly distributed throughout the cuboid and produce isolated volumes (∼2 to 7 μm in diameter) of partially reacted AlH 3. Eventually, the filaments spread over the entire internal substructure of the cuboid. However, although completely decomposed, the cuboid remains intact. Preexposure to 1.0 × 10 9 R 60Co γ rays blackens the surface and produces a "rougher" external surface with some densely decomposed patches and many more filaments. Additional patches containing denser clusters of acicular nuclei were observed in the surface after uv photolysis at room temperature and at 150°C. At 150°C delamination of the surface layer occasionally occurs in the coirradiated material. The increased number of nuclei formed by preirradiation and uv photolysis confirm kinetic studies which postulate the presence of additional nuclei formed by the irradiation.