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.