The role of hydrogen-containing species in the alkaline dissolution of aluminum was studied by secondary ion mass spectrometry (SLMS). Large number densities of submicron particles nucleated and then disappeared during dissolution, at intervals of approximately 3 min. The particles were composed of aluminum hydride, with an aluminum hydroxide surface layer. When particles first appeared, the aluminate ion concentration near the surface was at the solubility of Al(OH)3, and the potential was close to the Nernst potential for oxidation of AlH3 to Al(OH)3. The observed formation of AlH3 indicates that the dissolving Al surface was poised near this equilibrium potential, i. e. that a hydride species serves as an intermediate in the dissolution process.