The systematic microstructure and thermodynamic studies of the reactions between liquid aluminum and dense polycrystalline yttria (Y2O3) substrates at 1,273 K are reported in this article. The microstructure observations showed the presence of three reaction product zones extending similar to 1 mm into the oxide substrate of typical C4 (Co-Continuous-Ceramic-Composites) structure. The first zone starting from the drop side was composed of fine crystalline precipitates of the Al5Y3O12 (YAG) phase dispersed in the Al3Y matrix. The second zone was built of larger AlYO3 (YAP) crystals. The third zone formed elongated oxide precipitates (YAP) surrounded by the Al2Y intermetallic channels. The thermodynamic calculation indicated that, depending on the amount of the yttrium dissolved in aluminum, the YAG (up to 5 at.% Y), YAP (5-13 at.% Y), or Al2Y for higher content of yttrium might form at 1,273 K, while the Al3Y phase might appear during cooling.