BoxFerrous oxide activity has been determined in molten synthetic slag mixtures of the quaternary system FeO-TiO.-CaO-AI.03 at constant levels of Al,03' Furthermore, in the course of determining the FeO activity in this quaternary system, the ternary system FeO-CaO-AI=0, was also investigated at 1 450'C.The technique used to measure the ferrous oxide activity in the investigated systems was the well established one of gas-slag-metal equilibration in which molten slags contained in armco iron crucibles are exposed to a flowing gas mixture with a known oxygen potential until equilibrium has been attained.After equilibration, the final chemical analysis of the slags give compositions having a particular ferrous oxide activity corresponding to the oxygen potential of the gas mixture.The effect of Al,O, additions on the FeO activity was determined. The results are presented as a series of iso-activity curves on pseudo-ternary diagrams. The interdependence between the ferrous oxide activities and slag composition was also studied.