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The composition of the passive film on Ti-15Mo alloy, formed in 1 mol]L and 4mol/L HCI and 2 mol/L H2SO 4 solutions at 70 *C (160 OF) under anodic polarization is investigated by XPS and electrochemical techniques. Anodic polarization potential is found to have an obvious influence on the content of Mo in the film. At lower anodic polarization potentials, the surface of the passive film is enriched in Mo. However, at higher anodic polarization potentials, the surface is diluted in Mo. The anions of the electrolytes influence the composition of the passive film. In HCI solution, chloride ions are incorporated with the passive film during its formation. The passive film consists of a compound containing chloride and oxide ions. While in H2SO 4 solution, the passive film only consists of titanium-molybdenum oxide. Sulfide ions and other sulfur are not incorporated. Ti-Mo alloys have a better passivity than pure Ti in HC! and H2SO 4 solutions. This passivity is related to the enrichment of Mo in the surface of the passive film.