Thermogravimetric analysis (TGA) testing was used to measure the change in weight of polished samples of Al-XSi (X = 0 and 1.2 mass%) alloys. The samples were heated at 843 K for 6 h in dry air or nitrogen gas. X-ray diffraction was used to monitor the formation of the oxide films on the surface of the samples. The surface oxide films were more compact after the Al alloy samples were heated in air, and the oxide films showed some cracks after being heated in nitrogen gas. The thermally formed surface oxide films on the Al-1.2 mass% Si alloy samples heated in air and in nitrogen gas possessed loose structures, which comprised mainly γ-alumina, diaspore, and gibbsite, along with metallic silicon and/or aluminum. The weight variation curve of the films appeared serrated; this can be attributed to chain reactions (3Si + 3O 2 → 3SiO 2 + 4Al → 3Si + 2Al 2O 3) that occurred within the film.