Abstract: The dissolution kinetics of Al₂O₃ in CaO–Al₂O₃–SiO₂ slags was studied using a high-temperature confocal scanning laser microscope at 1773 to 1873 K. The results show that the controlling step during the Al₂O₃ dissolution was the diffusion in molten slag. It was found that the dissolution curves of Al₂O₃ particles were hardly agreed with the traditional boundary layer diffusion model with the increase of the CaO/Al₂O₃ ratio of slag. A modified diffusion equation considering slag viscosity was developed to study the dissolution mechanism of Al₂O₃ in slag. Diffusion coefficients of Al₂O₃ in slag were calculated as 2.8 × 10⁻¹⁰ to 4.1 × 10⁻¹⁰ m²/s at the temperature of 1773–1873 K. The dissolution rate of Al₂O₃ increased with higher temperature, CaO/Al₂O₃, and particle size. A new model was shown to be v_\mathrmA\mathrml_2\mathrmO_3=0.16\times r_0^1.58\times x^3.52\times \left(T-T_\mathrmm\mathrmp\right)^1.11 to predict the dissolution rate and the total dissolution time of Al₂O₃ inclusions with various sizes, where v_\mathrmA\mathrml_2\mathrmO_3 is the dissolution rate of Al₂O₃ in volume, μm³/s; x is the value of CaO/Al₂O₃ mass ratio; R₀ is the initial radius of Al₂O₃, μm; T is the temperature, K; T_mp is the melting point of slag, K.