We have investigated oxidation process of hydrogen terminated Si surfaces by oxygen plasma using infrared absorption spectroscopy (IRAs) in the multiple internal reflection (MIR) geometry. We have measured IRAs spectra in the Si-H stretching vibration regions of the hydrogen-fluoride acid (HF) treated Si surface and the hydrogen (H) plasma treated Si(100) surface during the oxygen plasma exposure in order to elucidate a plasma oxidation process. IRAs data demonstrated that the densities of hydride species rapidly decrease on the HF-treated surface by the oxygen plasma exposure;, on the other hand, they slowly decrease on the H-plasma treated surface by the exposure. IRAs data also demonstrated that dihydride (SiH2) species are more rapidly removed on the HF-treated surface by the oxygen plasma exposure than monoohydride (SiH) species. More hydride species are left on the H-plasma treated surface after the exposure to oxygen plasma than on the HF-treated surface because the H-plasma treatment makes Si surfaces rougher and more hydrogen insert into the subsurface regions than the HE treatment does. Oxygen plasma is hard to oxidize the H-plasma treated surface compared with the HF-treated surface. (C) 2004 Elsevier B.V. All rights reserved.