The dissolution behavior of Al₂O₃ and MgO particles in various CaO–Al₂O₃–SiO₂ slags was investigated using a confocal scanning laser microscope at 1 550°C. The dissolution paths of Al₂O₃ and MgO particles varied depending on slag compositions. Reaction products such as CaO·6Al₂O₃, CaO·2Al₂O₃ and Ca₂Al₂SiO₇ formed on the surface of Al₂O₃ particle. Surprisingly, however, in the case where reaction products were MgAl₂O₄ and Ca₂SiO₄ phases, these reaction products formed ring-like structure around MgO particles. Inside of the ring-like structure, liquid phases of which compositions correspond to the co-saturation compositions with MgO and MgAl₂O₄ (Ca₂SiO₄) phases. In addition, this unique dissolution phenomenon retarded the dissolution rate of MgO particle in molten slags. In the present study, the dissolution mechanism of this unique dissolution phenomenon is proposed.