We have explored phase relations in the fundamental ternary system MgO-FeO-SiO2 at lower mantle pressure (26 GPa) via synthesis experiments on the join MgSiO3-FeSiO3. The maximum solubility of FeO in (Mg,Fe)SiO3 perovskite as a function of temperature has been determined in the multianvil apparatus in the temperature range of 1150--1740 ¿C. Pure perovskites with 10 and 15 mol % FeSiO3 can only be synthesized above 1525 ¿C and 2072 ¿C, respectively, at 26 GPa. Synthesis experiments were also conducted in a laser-heated diamond anvil cell to resolve a previous discrepancy concerning the maximum solubility of FeO in (Mg,Fe)SiO3 perovskite determined by different high-pressure apparatus. Both M¿ssbauer spectroscopic and X ray diffraction studies revealed that laser-heated samples with more than 10 mol % FeSiO3 invariably contained magnesiow¿stite. The coexistence of magnesiow¿stite with relatively lower FeO content perovskite results from chemical disequilibrium process caused by the large temperature gradient in the laser-heated samples. Finally, we conclude that the maximum FeO content at the top of the lower mantle is about 12 mol % for a perovskite-dominated lower mantle, along a 2000-K lower mantle geotherm. ¿ American Geophysical Union 1996