The dissolution of alumina spheres in a synthetic CaO-Al2O3-SiO2 (CAS) slag was investigated in situ with a confocal scanning laser microscope (CSLM) in the temperature range between 1470 degrees and 1630 degrees C. Through image analysis, the evolution of the apparent particle radius is obtained as a function of time, exhibiting a clear temperature dependence. The rate-limiting step of the dissolution process is first evaluated with respect to the classical shrinking core model. Neither the boundary layer diffusion nor the chemical reaction in this model can fully explain the dissolution phenomena. By comparing the experimental results with numerical simulation, the rate-limiting step was shown to be diffusion in the majority of the tests. Effective binary diffusion coefficients were calculated.