Understanding the dissolution kinetics of non-metallic inclusions in liquid slag is key in optimization of slag composition for inclusion removal. In this study, the rate of dissolution of high-precision spheres of sapphire and alumina–magnesia particles in CaO–SiO 2 –Al 2 O 3 liquid slags was measured in situ using a laser scanning confocal microscope at 1500 °C. It was found that the rate of dissolution of both sapphire and alumina–magnesia particles increased when the slag basicity is increased. A layer was observed around the dissolving sapphire. This layer may be a product layer and/or indicative of a mass transfer rate-controlling system. In the case of alumina–magnesia particle, the kinetics appeared more complex and depended on slag composition. No product layer or mass transfer layer was observed around the particle dissolving in slag with low basicity, whereas for the high basicity slag, a product or stagnant layer was observed, similar to that of the sapphire particle. Assuming a mass transfer-controlled system, measured diffusion coefficients for sapphire particles in slags tested in this study ranged from 10 −11 to 10 −10 m 2 s −1 at 1500 °C.