Cr-Mo alloys prepared by sputter deposition are composed of a single BCC solid solution. The corrosion behavior of binary Cr-Mo alloys in 12 M HCl solution was studied by electrochemical measurements and X-ray photo-electron spectroscopy (XPS) analysis. The sputter-deposited Cr-Mo alloys are spontaneously passive in 12 M HCl solution. Their corrosion resistance is significantly high and increases with increasing alloy molybdenum content. XPS analysis reveals that the spontaneously passivated film on the alloys with < 50 at% molybdenum is rich in chromium while that on the alloys with > 50 at% molybdenum is rich in molybdenum. The angle-dependent XPS analysis shows that the film is not homogeneous but that the chromium content increases from the interior to the exterior of the film, and that the film changes from the interior oxide to the exterior oxyhydroxide. The outer chromium-enriched oxyhydroxide on the high chromium alloys is responsible for spontaneous passivation because of a high activity for cathodic oxygen and proton reductions.