Abstract: The oxygen probe with solid electrolyte was used for the determination of the activities of Fe_xO in basic complex slags, and the error analysis has been done. The activities of Fe_xO show positive deviation from Raoult's law. It is found that the deviation decreases with increasing CaF₂·Fe_xO content and the basity of slags, and increases with increasing P₂O₅ content. Activity of Fe_xO decreases with increasing temperature when the basity (CaO/SiO₂) value is greater than 2.1, Iso-activity curves of Fe_xO are drawn on the pseudo-ternary diagram Fe_xO-(SiO₂ + P₂O₅ +Al₂O₃)-(CaO+ MgO+MnO+CaF₂).