Phase composition and microstructure of isothermally solidified YAG/spinel (or Al2O3-rich solid solution) composites were investigated. It was verified that an equilibrium binary eutectic phase region consisting of YAG and Al2O3-rich spinel solid solution exists in the YAG-spinel-Alumina ternary system. Moreover, it was identified that a metastable system, YAlO3 and Al2O3-rich spinel solid solution, could be formed during rapid cooling from the eutectic temperature.