The cube-shaped phase (cube-phase) in an aged Al-Mg-Si alloy including Mg in excess has been investigated by an energy-filtering transmission electron microscope (EFTEM) equipped with an energy dispersive X-ray spectrometer (EDS) to determine its chemical composition. The quantitative chemical compositions of cube-phase and beta-phase (Mg2Si) were determined from EELS elemental maps. These data were also in good agreement with their EDS data. The cube-phase contained higher Mg content than the equilibrium beta-phase (Mg2Si), although they had the same crystal lattice of fcc, as reported by some previous researchers. (c) 2006 Springer Science + Business Media, Inc.