Four industrial aluminas, employed in the electrolytic smelting of aluminum, were found to exhibit quite differentrates of solution in cryolite. Physicochemical analysis showed that they all consisted of mixtures of varying proportionsof alpha-alumina and a number of varieties of transition aluminas; the alpha-alumina was also found to exhibit surfacedefects detectable by thermally stimulated exoelectron emission (TSEE), the number of which per unit area varied withthe sample. A mathematical model of the process of dissolution is proposed which is based on a number of assumptions,viz.: the transition aluminas, whatever their crystallographic species, are of the same particle size and have the same rateconstant; the alpha-alumina, the particle size of which is also independent of the sample, has a rate constant whichvaries in inverse proportion to the number of structural defects per unit area. Dissolution curves plotted from the theoreticaldata computed by means of the model were found to be comparable with those constructed from actual experimentalresults and both sets of curves ranked the aluminas investigated in the same order in terms of reactivity.