Fe3O4 has been studied by high-pressure diffraction to 43 GPa. No major changes in the spinel-type structure of magnetite is observed below 21.8 GPa. At higher pressure a sluggish transition to a high-pressure modification, h-Fe3O4, is observed. The X-ray diffraction pattern of the high-pressure modification is consistent with the orthorhombic unit cell (CaMn2O4-type structure, space group Pbcm) recently proposed for h-Fe3O4 by Fei et al. (1999), however, it is also consistent with a more symmetric CaTi2O4-type structure (space group Bbmm). Bulk modulus values for magnetite, KT0 = 217 (2) GPa, and h-Fe3O4, KT0 = 202 (7) GPa, are calculated from the pressure-volume data using a third-order Birch-Murnaghan equation of state. A thermodynamic analysis of the Fe-O system at high pressure is presented. The proposed equation of state of h-Fe3O4 gives an increased stability of wüstite relatively to a two-phase mixture of iron and h-Fe3O4 compared to earlier equations of state and removes an inconsistency in the thermodynamic description of the Fe-O system at high pressure.