Compressive creep tests in air have been performed on a polycrystalline submicron as sintered and slightly porous alpha-alumina material. Two different deformation mechanisms, depending on the applied stress and creep temperature, have been identified when the grain size becomes higher than a critical value < G*>. For low temperatures and/or low applied stresses, deformation occurs by grain boundary sliding accommodated by an in-series "interface reaction/diffusion of Al3+ cations'' process, with the limiting step being the interface reaction. In this case increased densification of the samples is observed after creep, compared to the as-sintered ones. In contrast, for high temperatures and/or high-applied stresses, deformation occurs by grain boundary sliding accommodated by the relocation and growth of preexisting cavities, the growth step being also controlled by the diffusion of Al3+ cations. In this case, a marked decrease of the relative density is measured on the crept samples compared to the as-sintered ones. Using these results, it is possible to identify the optimal conditions for superplastic forming of previously as-sintered parts, leading to shaped objects with an increased final density.