The mechanisms controlling an aluminum-water steam explosion and the possibility that a significant chemical reaction could be initiated have been debated for decades. This paper investigates the influence of hydrogen gas that is generated by the steam oxidation reaction. Most of this gas diffuses to the surface, but some diffuses into the molten metal. Analyses show that at elevated aluminum temperatures sufficient hydrogen is formed to saturate the diffusion layer propagating into the liquid metal, even considering that the hydrogen solubility increases significantly with temperature. If a steam explosion is initiated, the local rapid surface cooling would cause the dissolved hydrogen to become highly supersaturated, such that it would nucleate into high pressure gas bubbles within the locally cooled outer surface of the molten aluminum globules. This high pressure source would strip a thin molten layer, which has the thickness of the cooled thermal boundary, off of the surface as fine fragments that can oxidize explosively in the surrounding environment. Based on this mechanism, a methodology has been developed and found to be in agreement with the available large-scale data regarding (a) the conditions required for the occurrence of a significant chemical component in the explosion and (b) the energy releases that occur when a steam explosion initiates a chemical explosion.