Phase relations and physical properties of sulfur-bearing Fe systems under high pressure and temperature are reviewed with implications for the cores of the Earth, Mars, and the Moon for which planetary core seismic data are available. From geochemical arguments, the cores of the Earth and Mars contain up to 2 and 17 wt% sulfur, respectively, if the planet bulk compositions are chondritic and if core formation occurred under equilibrium conditions. We explore the behavior of these amounts of sulfur in these cores from a geophysical point of view by comparing the density and/or seismic velocity of the liquid metal alloys with seismologically observed values. Small amounts of sulfur would not have any significant effect on the properties of the Earth’s outer core at 136 to 330 GPa, which implies that 2 wt% sulfur can be present there when the Si-O-bearing Fe system already represents the density and VP of the outer core. Therefore, the Fe–Si–O–S system is a very promising candidate for Earth’s outer core. The assessment of the density of an Fe3S (Fe-16 wt% S) liquid is critically important for modelling the Martian core composition, but currently it is quite model dependent under Martian core pressures (20–40 GPa) partly because Fe-S liquids are non-ideal solutions. The effects of the addition of Ni to an Fe liquid should be examined since the presence of Ni could have significant impacts on the nonideality of Fe-S liquids. The physical properties of Fe–S liquids are consistent with the modelled seismic velocity but not with the density of the Lunar core. Reconciliation of mineral physics data and seismological observations on the core will provide a basis for establishing a consistent interior model for a remote telluric body. Holistic modelling of the systems Fe-S and Fe–Ni–S, including the Gibbs free energies and equations of state of constituent phases, will bring a better understanding of the origins and evolution of the cores of the neighboring terrestrial planets and the Moon.