In this work, we investigate the effects of powder chemistry on the sintering of MgO-doped specialty alumina. The stages at which MgO influences densification of Al2O3 were identified by comparing dilatometry measurements and the sintering kinetics of MgO-free and MgO-doped specialty alumina powders. MgO is observed to reduce the grain boundary thickness during densification using TEM. We show that MgO increases the solubility of SiO2 in alumina grains near the boundaries using EDS. First-principles DFT calculations demonstrate that the co-dissolution of MgO and SiO2 in alumina is thermodynamically favored over the dissolution of MgO or SiO2 individually in alumina. This study experimentally demonstrates for the first time that removal of SiO2 from the grain boundaries is a key process by which MgO enhances the sintering of alumina.