This study presents a thermodynamic analysis of slag refining in resulfurized automotive steels (RAS) within the CaO–Al 2 O 3 –SiO 2 –10 wt% MgO system. The combined effects of slag basicity, oxygen potential, refractory reactivity, and steel chemistry are evaluated. Equilibrium calculations identify the sources of dissolved aluminum, magnesium, and calcium in silicon‐killed steels and assess their impact on spinel, sulfide, and oxysulfide inclusions. The results show that alumina exhibits amphoteric behavior. In basic slags, it increases the equilibrium oxygen content, while in acidic slags, it acts as a weak deoxidizer. As slag basicity increases, the reduction of MgO and CaO by silicon is enhanced, leading to higher levels of dissolved magnesium and calcium and promoting the formation of magnesium aluminate spinel. However, even trace amounts of dissolved calcium can destabilize spinel, shifting stability toward calcium aluminates. Moreover, refining reactivity is strongly dependent on the steel grade, as alloying elements—particularly molybdenum—can influence silicon activity. Neutral or weakly basic slags with moderate alumina content and approximately 10 wt% MgO create the most favorable thermodynamic conditions for limiting the precipitation of spinel and CaS during ladle‐furnace refining.