A magnesia-alumina spinel phase (MgAlSP) and a magnesium-aluminum oxynitride phase (MgAlON) were formed in the closed pores of Al-MgO composite materials after the composites were annealed at high temperature in an oxidizing atmosphere. The MgAlSP and MgAlON phases were detected and investigated by X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry. The results indicated that the MgAlON phase first formed nanoscale particles on the surface of the spinel phase. These particles then interconnected with each other to form particle chain structures. With increasing temperature, the sizes of the MgAlON nanoparticles gradually increased. However, when the temperature was increased to 1550 degrees C, the chain structure of the particles decomposed and formed a granular MgAlON phase. (C) 2017 Published by Elsevier B.V.