The isothermal phase relations for the CaO–AlO_1.5–CeO_1.5 system have been investigated at 1773 K by a chemical equilibration technique. Activity of AlO_1.5 in this system was also measured by equilibrating molten Cu and the CaO–AlO_1.5–CeO_1.5 flux under a controlled oxygen partial pressure. The activity coefficient of Al at infinite dilution in molten Cu was determined at 1773 K. Iso-activity curves of AlO_1.5 for the CaO–AlO_1.5–CeO_1.5 system are drawn from the experimental results. The addition of CeO_1.5 to the CaO–AlO_1.5 system was found to decrease the activity coefficient of AlO_1.5 in the melts. The effect of Ce addition to Al-killed steel was also discussed using the present results.