Abstract: The dissolution of TiO₂ in the CaO-Al₂O₃-SiO₂ slag under static conditions was studied in the temperature range from 1643 K to 1703 K. After TiO₂ dissolved, the microstructure of the interface between TiO₂ and the slag was observed by scanning electron microscopy, and the concentration profiles of Ti⁴⁺ and other ions across the TiO₂/slag interfaces were analyzed by energy-dispersive X-ray spectroscopy. On the basis of these results, the dissolution behavior of TiO₂ was evaluated, and the diffusivity of Ti⁴⁺ in the bulk slag was estimated. According to the Stokes-Einstein relation, the viscosity calculated by a previously reported model gave a diffusivity of Ti⁴⁺ ions greater than that estimated by the concentration profiles of Ti⁴⁺ ions. The mechanism of TiO₂ dissolution in the CaO-Al₂O₃-SiO₂ slag is discussed in detail.