Melting experiments on Fe3S were conducted to 75 GPa and 2800 K in laser-heated and internally resistive-heated diamond anvil cells with in-situ x-ray diffraction and/or post-mortem textural observation. From the constrained melting curve, we assessed the thermal equation of state for Fe3S liquid. Then we constructed a thermodynamic model of melting of the system Fe–Fe3S including the eutectic relation under high pressures based on our new experimental data. The mixing properties of Fe–S liquids under high pressures were evaluated in order to account for existing experimental data on eutectic temperature. The results demonstrate that the mixing of Fe and S liquids are nonideal at any core pressure. The calculated sulphur content in eutectic point decreases with increasing pressure to 120 GPa and is fairly constant of 8 wt% at greater pressures. From the Gibbs free energy, we derived the parameters to calculate the crystallising point of an Fe–S core and its isentrope, and then we calculated the density and the longitudinal seismic wave velocity (Vp) of these liquids along each isentrope. While Fe3S liquid can account for the seismologically constrained density and Vp profiles over the outer core, the density of the precipitating phase is too low for the inner core. On the other hand, a hypothetical Fe–S liquid core with a bulk composition on the Fe-rich side of the eutectic point cannot represent the density and Vp profiles of the Earth’s outer core. Therefore, Earth’s core cannot be approximated by the system Fe–S and it should include another light element.