We have investigated the initial stages of UV ozone oxidation of hydrogen-terminated Si(100) and (111) surfaces using infrared spectroscopy in the multiple internal reflection geometry. Spectral features due to intermediate oxidation species such as SiH2(O2) and SiH(O3), which are generated by the attack of the back bonds of a surface Si atom by oxygen, are clearly observed. Upon UV ozone oxidation the concentration of the intermediate oxidation species initially increases and then drops, while that of surface hydride species Si–Hx decreases with increasing the UV ozone exposure, which indicates that at initial stages of oxidation oxygen attacks both of the Si–H and back bonds of a Si atom in the outermost layer. We find that the generation of the intermediate oxidation species exhibits a strong crystallographic orientation dependence; both SiH2(O2) and SiH(O3) are generated on the Si(100) surface, while the generation of the SiH2(O2) species is suppressed on the Si(111) surface.