As a fundamental study of slags in metallurgical processes, liquidus lines in the FeO(wüstite)–CaO–SiO₂ and Fe₃O₄(spinel)–CaO–SiO₂ systems were determined at temperatures between 1573 and 1673 K and partial pressures of oxygen between 10⁻⁴ and 36 Pa using a quenching method with a crucible of platinum foil. It was found that the two phases (solid wüstite and liquid) region in the FeO–CaO–SiO₂ system (1573 K) increased remarkably by changing the partial pressure of oxygen from 10⁻⁴ to 10⁻³ Pa. It was also found that the two phases (solid spinel and liquid) region in the Fe₃O₄–CaO–SiO₂ system (1573 K) at PO₂ of 10⁻² Pa was a little larger than that in the FeO–CaO–SiO₂ system at PO₂ of 10⁻³ Pa but it was extended considerably when the oxygen pressure was increased to 10⁻¹ Pa. However, the two liquids region did no change too much in the range of oxygen pressure above 10⁻¹ Pa even though the liquidus line became almost parallel to the Fe₃O₄–CaO axis in the ternary diagram when the oxygen pressure was increased. Furthermore, it was found that the two phase region in the Fe₃O₄–CaO–SiO₂ system at PO₂ of 10⁻¹ Pa decreased remarkably with increasing temperature. On the basis of the obtained and reported data, the homogeneous liquid regions in the FeO_X–CaO–SiO₂ systems (1573 K) at PO₂ of 10⁻³ Pa, 10⁻¹ Pa and in air were compared.