Phase relations in the system CaO–chromium oxide–SiO2were determined (a) in equilibrium with metallic chromium and (b) at various oxygen pressures in the range of ∼ 10−10to 10−13atm. Two ternary phases occur on the liquidus surface in equilibrium with metallic chromium, viz, the garnet Ca3Cr2Si3O12(uvarovite), and CaCrSi4O10of the gillespite‐type structure and in which essentially all the chromium is present in the divalent state (Cr2+). Furthermore, a previously reported binary calcium chromite phase “X” of approximate composition (Ca0.4Cr0.6)Cr2O4is stable also on the liquidus surface of the present system. The lowest liquidus and solidus temperatures in the system under these conditions (contact with metallic chromium) are significantly lower (by 100° to 200°C, depending on CaO/SiO2ratios of the melts) than those for the same system in air. Furthermore, there is considerable solid solution of chromium oxide in lime (CaO) and the various crystalline calcium silicates (pseudowollastonite, CaSiO3; rankinite, Ca3Si2O7; dicalcium silicate, Ca2SiO4; tricalcium silicate, Ca3SiO5), presumably by partial substitution of Cr2+for Ca2+toward hypothetical chromium analogs of each CaO‐containing phase, particularly in lime and Ca2SiO4. At the somewhat higher oxygen pressures imposed by the CO2─H2gas mixtures of CO2/H2ratios of 1/24 and 1/8.1, the phases containing most of the chromium in the divalent state (CaCrSi4O10and (Ca0.4Cr0.6)Cr2O4) do not appear on the liquidus surface, the primary phase area of garnet (uvarovite) decreases, the lowest liquidus and solidus temperatures increase noticeably (−50° to 100°C), and the amounts of chromium substituting in lime and the various crystalline calcium silicate phases (CaSiO3, Ca3Si2O7, Ca2SiO4, and Ca3SiO5) decrease considerably from those observed for the system in equilibrium with metallic chromium. Combination of the data obtained under these reducing conditions with those previously reported for the system under more oxidizing conditions permits derivation of the general features of phase relations in the quaternary