Abstract: Desulfurization of CaO–Al₂O₃ particles in molten steel was observed in situ using high-temperature confocal scanning laser microscopy. The effects of the aluminum and silicon contents of molten steel on desulfurization were analyzed. When the total aluminum content in the steel increased from 6 to 1100 ppm, the CaS content in CaO–Al₂O₃ particles increased from 2.1wt% to 84.84wt% after the reaction for 90 s. Furthermore, when the silicon content in the steel increased from 0.01wt% to 2.20wt%, the CaS content in CaO–Al₂O₃ particles increased from 1.53wt% to 79.01wt% after the reaction for 90 s. This indicates that the increase in the aluminum and silicon contents of the steel promoted the desulfurization of CaO–Al₂O₃ particles. A kinetic model was established to predict the CaO–Al₂O₃ particles composition, and the diffusion coefficient of sulfur in CaO–Al₂O₃ particles was 9.375 × 10^-10 m²·s^-1 at 1600°C, which provided a new method for the calculation of diffusion coefficient.