The breakaway behavior of surface oxide film on Al–Si–Mg alloy powder particles was investigated by X-ray photoelectron spectroscopy using synchrotron radiation (SR-XPS), which is a topmost-surface analysis method with less than 1.5 nm analysis depth and high sensitivity. During the heating of the powders up to 823 K, the dissolved magnesium in the powder particles was concentrated in the surface oxide below 670 K, and saturated in the composition with a Mg/Al ratio of approximately 0.5. Over 670 K, the metallic aluminum appeared on the topmost surface of the particles. It was speculated that the surface reaction was due to the reduction of surface oxide film by the concentrated magnesium or the breakage of the surface oxide film by the crystallization and growth of MgAl₂O₄.