Seismic observations suggest that the uppermost region of Earth s liquid outer core is buoyant, with slower velocities than the bulk outer core. One possible mechanism for the formation of a stably stratified layer is immiscibility in molten iron alloy systems, which has yet to be demonstrated at core pressures. We find immiscibility between liquid Fe-Si and Fe-Si-O persisting to at least 140 GPa through a combination of laser-heated diamond-anvil cell experiments and first-principles molecular dynamics simulations. High-pressure immiscibility in the Fe-Si-O system may explain a stratified layer atop the outer core, complicate differentiation and evolution of the deep Earth, and affect the structure and intensity of Earth s magnetic field. Our results support silicon and oxygen as coexisting light elements in the core and suggest that SiO2 does not crystallize out of molten Fe-Si-O at the core-mantle boundary. © 2019 National Academy of Sciences. All rights reserved.