NASA’s InSight mission provided seismological constraints on the density of the Martian core which showed that the core is primarily liquid with a significant amount of light elements present. More recent analyses have indicated the possible presence of a solid inner core. From cosmochemical arguments, sulphur and oxygen have been widely accepted as promising candidates for the light elements in the Martian core. In order to examine the liquidus field of the system Fe-FeS-FeO, we have conducted high-pressure and -temperature experiments on the FeS-FeO join to 51 GPa. A comparison of thermodynamic calculations of the liquidus curves with the experimental data at about 20 GPa has demonstrated that the mixing properties of FeS-FeO liquids can be ideal mixing. Since the Fe-S liquids are greatly stabilised compared to Fe-FeO liquids (highly positively nonideal) and FeS-FeO liquids (ideal), the eutectic composition at 20 GPa should be placed near the Fe-S join, which confirmed the O-poor eutectic composition in the system Fe-FeS-FeO previously reported from phase stability experiments. From the eutectic composition, previously proposed Martian core compositions likely precipitate FeO or Fe3S in the liquid core. Further seismic data is expected to resolve the crystallographic information of the inner core phase, which will provide critical information about the Martian core composition.