The possibility is considered that observed underwater, steam explosion-induced ignitions of low-temperature molten aluminum fragments (microspheres) can be explained by Ostwalds [1] “law of stages” in nucleation processes. That is, it is proposed that the chemical change from molten aluminum to solid aluminum oxide beneath the surfaces of the microspheres proceeds in three distinct stages: (i) chemical reaction between a dissolved species of oxygen and molten aluminum to metastable molten aluminum oxide, (ii) nucleation of solid aluminum oxide crystals within the metastable melt, and (iii) the growth of the crystals into a continuous scale of solid aluminum oxide. During Stages (i) and (ii) the rate at which oxygen is adsorbed by the growing aluminum oxide melt layer is believed to be sufficiently high to support the rapid oxidation of the melt fragments within the steam explosion zone. Because the oxygen adsorption potential in solids is much lower than in liquids, the oxidation rate at the end of Stage (iii) is expected to decrease discontinuously by several orders of magnitude, leading to a final stage of net microsphere cooling. Thus, according to the theory presented herein, underwater aluminum ignition is determined by the outcome of a race between the aluminum chemical reaction rate and the rate of aluminum oxide crystallization. The present theoretical approach, primarily concerned with trends and order-of-magnitude predictions, indicates that, unless the initial temperature of the aluminum melt is well above its melting temperature, a very strong initiator (e.g. blasting cap) is required in order for chemical energy to be released during an aluminum/water explosion. With further development, the reaction/crystallization model may prove useful for guiding future experimental work in this area.