Mg-Fe partitioning between coexisting phases, magnesiow¿stite (Mw) and olivine (α), Mw and β- phase, Mw and spinel (&ggr;), and Mw and perovskite (Pv), has been determined experimentally with piston-cylinder apparatus, the multianvil device and the diamond anvil cell technique at pressures between 2 and 28 GPa and temperatures between 1473 and 1773 K. The solution parameters of each solid solution were obtained by fitting the experimental data simultaneously using the Margules formulation. The optimized solution parameters (in J/mol) are WMwMg-Fe=16100, WMwFe-Mg=26300-5.56T; WαMg-Fe=4500+130P, WαFe-Mg=6500+130P; WβMg-Fe=1000, WβFe-Mg=2000; W&ggr;Mg-Fe=900-1.10T, W&ggr;Fe-Mg=3900; and WPvMg-Fe=4130-1.37T+110P, WPvFe-Mg=-4050-2.45T+150P where P is in GPa and T in K. These parameters are consistent with solution calorimetry and phase equilibrium data. Phase relations in the MgO-FeO-SiO2 system were calculated by using the Margules solution model combined with internally consistent thermodynamic data of the pure phases. The computation provides independent constraints for phase relations in the system, in addition to those directly determined in P-T-Xi space. ¿1991 American Geophysical Union