Synthetic spinel (MgAl2O4) has been found to disproportionate into a mixture of its component oxides, periclase and corundum, at a loading pressure of about 180 kbar and between 1000 and 1400°C. The polycrystalline powder sample was compressed in a diamond‐anvil cell and heated by a continuous YAG laser. The exact transition pressure is not known due to transient increases in pressure during the local and rapid heating process. The phase boundary, however, has been calculated to be P(kbar) = (137±8) + 0.009T(°C) on the basis of the available thermochemical data and compression curves for MgAl2O4, MgO, and A