Oxide films entrapped in Al-XSi alloys with an X from 0% to 13% are different, and can be identified by the presented ultrasonic-vibration treatment. After polishing and ultrasonic-vibration treatment, the surfaces or castings then show differently shaped foggy marks, including lumps, flakes, strips or spots. Oxide films fracture and particles become detached from the film during ultrasonic-vibration treatment. The polished surface thus becomes partly eroded after treatment, and these eroded areas are visibly as differently shaped foggy marks. This paper presents a sequential summary of the formation of these eroded areas, foggy marks, in samples of pure aluminum and Al-XSi alloys during ultrasonic-vibration treatment.