In the determination of activities of components other than SiO₂ for the CaO–SiO₂–AlO_1.5–MgO slags equilibrated with molten Si alloys, activities of SiO₂ in the slags are required and their accuracy will considerably affect the results. For the CaO–SiO₂, CaO–SiO₂ –AlO_1.5 and CaO–SiO₂ –MgO slags, the activities of SiO₂ were obtained from the distribution of each element between slags and metals, and each thermodynamic property in molten Si alloys at 1823 and 1873 K using a newly developed method with the Gibbs–Duhem equation. The data obtained in the present research were compared with those reported by other investigators, and showed reasonable agreement in wide slag composition ranges. These activity data of SiO₂ will be employed in the subsequent determination of other component activities for the binary and ternary slag systems concerned.