Behavior of F and O on the Si(100) surfaces dipped in aqueous solution of HF was examined by use of X-ray photoelectron spectroscopy (XPS). The F 1 s spectrum observed was separated into two Gaussian peaks with 685.6eV and 687.4eV binding energies, which are considered to correspond to Si-F and Si-F₂ bonding respectively from the difference in the amount of negative charge of each F. Peak intensity of Si-F increased with increasing HF concentration of the solution. On the other hand, the variation of peak intensity of Si-F₂ with HF concentration was small. In the F 1 s spectra from the Si(100) surfaces exposed to the atmosphere after dipping in aqueous solution of HF, peak intensity of Si-F decreased with increasing the exposing time, but that of Si-F₂ hardly varied with the exposing time. These experimental results were explained in terms of the differences in the chemical bonding state between Si and F.