Under shock-wave compression, Fe1-xO undergoes a transition to a dense metallic phase at pressures near 70 GPa. The geochemical significance of this transition has been unclear. Here, first-principles electronic structure calculations (using the FLAPW method and GGA exchange-correlation) show that the shock-wave discontinuity of FeO results from a RB1 (rhombohedrally distorted NaCl structure) to B8 (NiAs structure) transition. The metallic nature of the FeO (B8) phase is argued to result from a breakdown of the Mott insulating condition, rather than an Fe(3d)-O(2p) gap closure. As such, the metallization of FeO is probably not a basis for invoking oxygen in the Earth's core. The stability of FeO(B8) over FeO (RB1) at high pressure is comparable to the ideal -TΔS of mixing of FeO in (Mg,Fe)O at mantle temperatures. Consequently, it is uncertain if FeO(B8) is present as a separate phase in the Earth's interior. ¿ American Geophysical Union 1995