In situ synchrotron X-ray powder diffraction measurements using a Paris-Edinburgh pressure cell were performed to investigate the nature of the high-pressure breakdown reaction of magnetite (Fe 3O 4). Refinement of diffraction patterns reveals that magnetite breaks down via a disproportionation reaction to Fe 4O 5 and hematite (Fe 2O 3) rather than undergoing an isochemical phase transition. This result, combined with literature data indicates (1) that this reaction occurs at -9.5-11 GPa and 973-1673 K, and (2) these two phases should recombine at yet higher pressures to produce an h-Fe 3O 4 phase.