<p>Constraints can be placed on the formation conditions of (Mg,Fe)_XO ferropericlase inclusions in natural diamonds based on their Fe³⁺ contents, given suitable knowledge of how these contents change with pressure, temperature, composition and oxygen fugacity. To achieve this, experiments have been conducted to determine the maximum Fe³⁺/Fe_tot ratio in ferropericlase, controlled by its coexistence with magnetite–magnesioferrite (Fe₃O₄–MgFe₂O₄) up to pressures of ~ 6 GPa, and with the high-pressure [Fe,Mg]₂Fe₂O₅ phase between 10 and 30 GPa. The experiments were performed between 1200 and 1800&#xa0;°C and across a range of bulk Fe/(Fe + Mg) ratios. Mössbauer and electron energy loss spectroscopy were used to determine ferropericlase Fe³⁺/Fe_tot ratios, a task that is complicated by exsolution of nanocrystalline magnetite–magnesioferrite during quenching. Using these data, a thermodynamic model was developed that describes the entire range of ferropericlase compositions in the Mg–Fe–O system between 1 atmosphere and 30 GPa. It is shown that the Fe³⁺/Fe_tot ratio of ferropericlase within diamond-forming assemblages decreases strongly above 10 GPa, and that the majority of ferropericlase inclusions in diamonds with measured Fe³⁺/Fe_tot, have ratios too high for them to have formed in the lower mantle. Most of these ferropericlase inclusions have maximum formation depths in the transition zone, but a few of the most oxidised could only have formed in the upper mantle from oxidised precursors such as carbonates. We also show that high pressure (Fe,Mg)₂Fe₂O₅ and (Fe,Mg)₃Fe₂O₆ phases can become stable during diamond formation towards the base of the transition zone, when bulk Fe/(Fe + Mg) ratios are &gt; 0.5.</p>