An investigation has been carried out to determine the formation conditions of Ce2O2S and CeAlO3 in cerium treated Al-killed steel. Argon protection melts have been manufactured of appropriate base steel compositions with varying additions of cerium (0.006-0.04%). The work has shown that 0.0015-0.02% may be retained in steels after Al deoxidation and cerium treatment while oxygen and sulphur content be 14-17 ppm and 60-70 ppm respectively. With 0.0099-0.02% retained cerium, resulting inclusions are largely spheroidal in shape and consist of several crystalline compounds, notably Ce2O2S and CeAlO3. Thermodynamic analysis indicates that the CeAlO3 would be formed with the cerium content in the range of 8.65ppb to 1.45ppm when the content of aluminum was 0.01%.