Abstract: Mold flux serves the crucial metallurgical function of absorbing inclusions, directly impacting the smoothness of the casting process as well as the cast slab quality. In this study, the dissolution behavior and mechanism of TiO₂ and TiN inclusions in molten CaO–SiO₂–B₂O₃-based fluorine-free mold flux were explored by in situ single hot thermocouple technology combined with X-ray photoelectron spectroscopy. The results showed that TiO₂ inclusions are effectively dissolved by the molten slag within 76 s, during which the original octahedral TiO₆⁸⁻ structures are destroyed and convert to the networker tetrahedral TiO₄⁴⁻ structures. However, the dissolution rate is much lower for TiN inclusions than for TiO₂ inclusions. This can be attributed to the fact that the TiN particles need to be oxidized and then dissolved in the molten slag to form tetrahedral TiO₄⁴⁻ and octahedral TiO₆⁸⁻ structures during the TiN inclusion dissolution process, which is accompanied by the generation of a large amount of N₂ gas. Moreover, CaTiO₃ crystals tend to nucleate and grow on bubble surfaces with sufficient octahedral TiO₆⁸⁻ structures and Ca²⁺ ions, eventually resulting in the molten slag being in a solid–liquid mixed state.