Liquidus and solidus phase relations have been determined for the system iron oxide–NiO–SiO2under strongly reducing conditions obtained by using CO2–CO gas mixtures in controlled proportions. The phase relations were determined with the well‐known quenching method: oxide mixtures were equilibrated in vertical tube resistance furnaces, followed by quenching to room temperature and identification of phases with transmitted‐ and reflected‐light microscopy and X‐ray diffraction. Three crystalline phases are present on the liquidus surface: olivine (Fe2SiO4–Ni2SiO4solid solutions), oxide (“FeO”–NiO solid solutions), and silica (tridymite or cristobalite, depending on temperature). The “ternary peritectic” point where these three phases coexist with liquid is at 1571°C, with a liquid composition of approximately 19 wt%“F