The pseudobinary system Al2O3-MgO has been studied in the temperature range 1750-2100 K by the Knudsen effusion method in combination with mass spectrometric analysis of the vapor phase. Over the entire composition range, except for the 100 mol % Al2O3 boundary, the vapor phase over the system consists of three species: Mg, O-2, and O. The partial pressures obtained have been used to construct a p-x section through the Al2O3-MgO phase diagram at 1900 K. The standard enthalpy of formation of the MgAl2O4 spinel at 298 K has been determined by third-law calculations:-2301.61 +/- 11.00 kJ/mol. We have derived equations for the temperature dependences of the partial pressures of the vapor species over the Al2O3-MgO system.