The structures of passivation films of sintered Ti-(0, 10, 20, 30, 40,100)mass%Mo alloys formed in 35%HCl, 3%NaCl and 103mol/m3(3.8%)NaOH solution were studied by X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectra of Mo3d, Ti2p and O1s of Ti-Mo alloys immersed in a 35%HCl solution showed that Mo4+/Mo6+ ratio of the passivation film increased with increasing Mo content in the alloy and that Mo in the passivation film was enriched by the preferential dissolution of Ti in this film. This showed that the presence of Mo4+ state in this film was very important for improving its corrosion resistance. From X-ray photoelectron spectra of Ti-Mo alloys immersed in a 3%NaCl solution, it was indicated that the Mo content in the passivation film was equal to that in the alloy and that the passivation film consisted of mixture of Mo6+ and Ti4+ states. On the other hand, X-ray photoelectron spectra of Ti-Mo alloys immersed in a 3.8%NaOH solution showed that Ti in the passivation film was enriched by the active preferential dissolution of Mo. It was thought that the corrosion resistance of this alloy in NaOH solution was improved because of the passivation film consisted of stable Ti oxides