Auger electron spectroscopy was used to study the segregation of magnesium and calcium to the prismatic plane of sapphire doped with Mg by ion implantation. Segregation behavior depended strongly upon the annealing atmosphere. Air annealing showed Mg segregation to the free surface whereas vacuum annealing resulted in no observable Mg segregation above the detectability limit of our instrument. This is attributed to excessive vaporization of MgO at low oxygen pressures. In the absence of Mg on the surface, strong Ca segregation was observed although the bulk concentration of Ca was below 40 ppm, the total impurity level. The effective heat of segregation of Mg in air was found to be about -1.4 eV in the temperature range 1300° to 1550°C. The Ca coverage on the surface did not follow a Langmuir plot, probably because of Ca–Ca interaction at the larger surface concentrations. The implication of these results to the understanding of transport-related properties of impurity-doped alumina is discussed.