It is a well-known that CaS precipitates on the CaO-Al 2 O 3 inclusion and deteriorate the corrosion resistance property of steel. To prevent the formation of CaS, the sulfide capacity of molten CaO-Al2O3 oxide, and the distribution of S between these molten oxides and molten steel have been studied. However, in some cases, CaS forms after the solidification of steel and oxides during heat treatment. To clarify the formation mechanism of CaS on the oxides at solid state, the solubility of S in the solid compounds of CaO-Al 2 O 3 system, and the S distribution behavior among these compounds and solid steel are important. In this study, CaO-Al 2 O 3 -CaS oxides with various compositions were melted at 1873 K under CO-CO 2 -SO 2 atmosphere, and then cooled and kept at a temperature range of 1373 K -1473 K followed by quenching. Afterwards, the formed solid compounds and the S content in each compound of the quenched oxide were observed by EPMA. The results showed that, irrespective of the holding temperature, the S content of solid C1 2 A 7 (C stands for CaO and A for Al 2 O 3 ) varied much and the highest values was approximately 1 mass% and was close to the S solubility of the liquid CaO-Al 2 O 3 system of the same composition at 1773K. The S contents in other solid compounds, which were CA 2 , CA, or C 3 A, were low. This result indicated that the S is soluble only in C1 2 A 7 in the solid state, and the precipitation of CaS from CaO-Al 2 O 3 oxide could be prevented by controlling the oxide composition.