Reoxidation of liquid steel containing Al and Ti by 14%CaO-35%Al₂O₃-10%MgO-41%SiO₂ (in mass%) slag was investigated at 1823 K with initial Al content of 820 mass ppm and Ti content varied from 100 to 500 mass ppm. It was observed that Al and Ti in the steel were simultaneously oxidized by SiO₂ in the slag, and the soluble oxygen was supersaturated during the course, particularly with respect to Al. Based on the experimental results, a new mechanism of the reoxidation reaction was proposed, which involves chemical reactions both at the metal/slag interface and in the bulk metal. Self-dissociation of SiO₂ into Si and O at the slag/metal interface was found to play an important role in both supersaturation of oxygen, and subsequent formation of complex oxide inclusions. Formation of inclusions having a two-layer structure where an Al₂O₃ core was enclosed by complex Al-Ti-O oxide was explained in relation with supersaturation of oxygen in the steel.