First-principles calculations of the structure, energetics, compressibility, and elastic constants of aluminous stishovite reveal that the dissolution of aluminum in stishovite can accommodate a significant population of both oxygen vacancies and hydrogen defects. The results show that the incorporation of aluminum in stishovite account for a 1.2(±0.1)% decrease in K ₀ per mole percent dissolved aluminum, independent of dissolution mechanism (hydrous or anhydrous). Elastic constants, calculated to be in agreement with experimentally-derived values, indicate that the rutile to CaCl₂ transition is likely to be unaffected by the dissolution of small amounts of aluminum and/or hydrogen. As a result, the dissolution of aluminum into stishovite in a subducting basaltic layer will have no more than a 0.2% effect on the density of that layer as it enters the lower mantle.