The chemical composition and binding state of calcium phosphate (CaP) thin films formed on silica substrates were investigated using X-ray photoelectron spectroscopy (XPS). The CaP films were prepared at various treatment periods (such as 15, 30, and 60 min) through contact with an ionic solution supersaturated with calcium and phosphate at 37 °C. These CaP films are denoted as CaP15, CaP30, and CaP60, respectively. Examination of XPS spectra demonstrated that carbon, oxygen, calcium, and phosphorus were the major elements constituting the CaP films. For the carbon and oxygen elements, the intensities of the XPS peaks decreased as the treatment time increased. On the other hand, the peak intensities of calcium and phosphorus increased after extended treatment, during which maturation of the CaP film occurred. The continuous variation of the Ca/P ratio of the CaP film illustrates that a variety of active chemical interactions existed. The value of the Ca/P ratio for CaP15 was 1.35, which is very similar to that of octacalcium phosphate. when treatment was continued for over 30 min, the Ca/P values declined to 1.27. Hence, the excess amount of phosphorus was incorporated into the CaP film during prolonged incubation to form calcium-deficient apatite-like structures. The peak analysis of high-resolution XPS spectra for C 1s, O 1s, Ca 2p, and P 2p demonstrates that the prepared films consist of calcium phosphate. For an examination of the applicability of our CaP preparation method to polymeric substrates, we formed CaP films on polystyrene. HOS cell adhesion to the CaP films appears to be significantly faster than that on the control, polystyrene. The increase in electrostatic interactions due to the highly charged nature of both the CaP films and the cell surface may be a plausible cause for this event. The proliferation of HOS cells was most pronounced for both CaP15 and CaP30. According to these experimental results, we believe that the CaP films introduced in this study can be applied for the surface modification of polymeric and metallic substances to improve the functionality of biomaterials.