Complementary structural characterization of the reaction product region formed due to high-temperature interaction (1273 K) between molten aluminum and dense polycrystalline yttria substrate was performed. The reaction product region extending up to 1 mm into the oxide substrate was characterized by a wavy shape morphology and multilayer structure consisted of three-layered zones. The application of transmission electron microscopy coupled with focused ion beam preparation technique allowed the detailed structural examination of reactively formed compounds and interfaces between different zones and phases. Fine crystalline precipitates of Al(5)Y(3)O(12) (YAG) phase surrounded by the Al(3)Y were detected within the first zone. The second layer consisted of much bigger AlYO(3) (YAP) crystals and the third one, which was the widest zone, revealed a typical C4 microstructure where elongated YAP precipitates accompanied the Al(2)Y metallic channels.