Diffusionless phase transformations in Fe-Ni alloys with up to 35 wt % Ni were studied using resistance-heated diamond anvil cells and synchrotron radiation. At temperatures up to 600 ¿C and pressures up to 25 GPa, Fe35%Ni showed only the face-centered cubic (fcc) phase while Fe25%Ni and Fe10%Ni showed the body-centered cubic (bcc) and hexagonal close-packed (hcp) phases in addition to the fcc phase. The phase relationships of these solid phases are topologically similar to those of pure iron. The triple point becomes lower in temperature, and the slope (dT/dP) of the fcc/hcp boundary decreased as Ni-content increases in the alloys. From our results we infer that Fe-Ni alloys in the Earth's inner core might exist as two phases, fcc and hcp. The existence of these two phases could have played an important role in the separation of the inner and outer core. ¿ American Geophysical Union 1988