1] The Fe 3+ /SFe ratios in both (Al, Fe)-bearing MgSiO 3 post-perovskite phase and Ca-ferrite-type Al-phase, synthesized in a natural mid-oceanic ridge basalt (MORB) composition at 113 GPa and 2240 K, were determined by electron energy-loss near-edge structure (ELNES) spectroscopy. The results demonstrate that post-perovskite has high proportions of Fe 3+ with Fe 3+ /SFe ratio of 0.65 ± 0.04. The high Fe 3+ concentration in post-perovskite may have significant effects on its physical properties, phase stability, and iron partitioning. In contrast, the Ca-ferrite-type Al-phase, which is the second Fe-bearing phase in a subducting MORB crust, is enriched in Fe 2+ rather than Fe 3+ with Fe 3+ /SFe ratio of 0.15 and 0.29.