We have used theoretical methods (Full-Potential Linear-muffin-tin-orbital, FPLMTO, and lattice dynamics) to calculate the Gibbs free energy of the phases, perovskite (MgSiO3), SBAD (SiO2), stishovite (SiO2) and periclase (MgO). At 300 K, perovskite is found to be stable at all pressures up to 150 GPa. As the temperature is increased, the free energy difference between the oxide mixture and perovskite decreases and at 3000 K and at pressures between 105 and 115 GPa, the oxide mixture becomes more stable. The calculated equilibrium pressure is much higher than the observed experimental pressure which may be due to stress in the sample and/or due to the inadequacy of the applied theoretical models not accounting for anharmonicity effects. ¿ 1998 American Geophysical Union