Reactions between molten alloys and solid silicates were investigated under reducing conditions, down to four log units below the IW buffer, at 20–25 GPa and 2000 °C in the Mg–Si–Fe–O–S system. The iron-rich alloys formed in the Fe–Si–S ternary system were single phased at high-pressure and temperature, suggesting closure of the liquid immiscibility gap at these conditions. At the metal/silicate interface, the reduction of Fe2+ was coupled to oxidation of Si in metal, a reaction also observed in similar sulphur-free systems. Silicon and sulphur were observed to coexist simultaneously in the metal phase in contact with silicates and magnesium sulphide, MgS, was observed in the experiments. This latter phase would not segregate to the planetary cores, thus providing a reservoir of sulphur in the silicate and surficial planetary envelopes.