Surface films formed on titanium specimens immersed in electrolyte solutions (pH 4.5, 5.2, 7.4) at 37 degrees C for 1 h, 1 d, 30 d, and/or 60 d, were characterized using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) to understand the reaction between titanium and inorganic ions. For comparison, the surface of Ti-6AI-4V and Ti-50Ni were also characterized. XPS data revealed that calcium phosphates were naturally formed on these specimens. In particular, compared with the calcium phosphates formed on the titanium alloys, the calcium phosphate formed on titanium immersed for 30 d in the solution with pH 7.4 was more like hydroxyapatite. The compositions of the calcium phosphates formed on the specimens changed with the immersion time and the pH value of the solution. The spectrum obtained using FTIR-RAS from titanium immersed in the solution with pH 7.4 for 60 d was similar to that obtained from carbonate-containing hydroxyapatite. The results indicate that a calcium phosphate similar to apatite is naturally formed on titanium in a neutral electrolyte solution in 30 d. In regard to titanium being a biomaterial, we found this to be an intriguing property. It is possible that this calcium phosphate is responsible for the resulting biocompatibility of titanium.