The interfacial characterization of Al alloy reinforced with Al₂O₃particulates was performed using X-ray diffraction and transmission electron microscopy. Layer of MgAl₂O₄ single crystals having octahedral shape was observed either at (Al₂O₃)_p/Al interface or near (Al₂O₃)_p. Based on the experimental observations and thermodynamic considerations, MgAl₂O₄ crystals formed at the surface of (Al₂O₃)_p are believed to grow by consuming Al₂O₃. Significant surface erosions of (Al₂O₃)_p around MgAl₂O₄ were also observed along a certain crystallographic orientation of (Al₂O₃)_p irrespective of the shape of MgAl₂O₄. Accompanying the interfacial reaction between Mg and Al₂O₃, reduction in precipates density was observed in the vicinity of Al₂O₃.