As the Earth accreted, metallic materials segregated from silicates to form the iron-rich core. The proportions of the refractory lithophile elements in the silicate part of the Earth are thought to have remained similar to that of chondrite meteorites throughout accretion1. However, although niobium(Nb) and tantalum (Ta) are both classified as refractory lithophile elements and share a similar degree of incompatibility in mineral structures2, the Nb/Ta ratio of the bulk silicate Earth is subchondritic3. To explain this paradox, it has been proposed that Nb becomes siderophile at the highpressures of core formation, and was preferentially removed from the silicate Earth4–6. Here we conduct metal/silicate partitioning experiments at a range of oxygen fugacities and show that Nb and Ta are both siderophile elements under reducing conditions, but become so at dierent oxygenfugacities, leading to fractionation. We find that pressure has a negligible influence on the Nb/Ta ratio. Applying our partitioning data to existing theoretical accretion models7,8, we reproduce the Nb/Ta ratios of the bulk silicate Earth, Mars and the dierentiated asteroid 4 Vesta, and discuss the implications for Moon formation. We conclude that planetary accretion of reduced materials played an important role in the chemical evolution of Earth and, more generally, that Nb and Ta can be used to trace prevailing oxygen fugacities during the segregation of planetary cores.