Sintering of magnesium aluminate spinel of the MgO-excess, stoichiometric, and Al2O3-excess compositions has been investigated under vacuum and in air for the effect of low oxygen partial pressures. Densification enhancement of the surface lager is due to MgO evaporation which generates oxygen vacancies in the host crystal lattice. Regions of different grain sizes are observed from samples sintered under both conditions, Microstructural features of pairwise breakup of particle chains representing differential sintering are characteristic of the less-densified sample interior. The densification improved initially and yet was retarded in the intermediate sintering stage when the density exceeded 75% with vacuum-sintering owing to MgO evaporation.