The spatial distribution of Mg solute in polycrystalline, single‐phase Al2O3has been obtained, using a secondary ion imaging technique. Segregation of Mg to the Al2O3grain boundaries is clearly observed—strong evidence that the principal role of MgO is to reduce the grain boundary mobility via a solute‐drag mechanism. In addition, Mg (dopant) and Ca (impurity) are shown to cosegregate, shedding further light on magnesium’s ability to stabilize microstructural evolution in Al2O3. ©1995 American Institute of Physics.