High temperature confocal scanning laser microscopy (HT-CSLM) is used to study the dissolution behavior of Al2O3 inclusions in various slag compositions in the system CaO-Al2O3-SiO2-MgO. This method enables the in situ observation of the dissolution at steelmaking temperatures. The change of the diameter of the spherical inclusion is measured by image analysis of pictures obtained from the HT-CSLM. Subsequently, dissolution rates and normalized dissolution curves are determined, and the governing dissolution mechanism is identified by the use of a modified approach of the diffusion equation introduced by Feichtinger et al.27 and compared with the dissolution of SiO2 previously reported by the same authors.27 Finally, effective binary diffusion coefficients are calculated. Slag viscosity is shown to essentially affect the dissolution behavior, changing the normalized dissolution pattern from rather S-shaped (high slag viscosity) to a parabolic form (low slag viscosity).