The oxidation process of porous silicon in wet air at 323 K was investigated by infrared spectroscopy, particularly focusing on the behavior of the stretching vibrations of Si-H. The fine structure of the absorption bands were clarified with help of vibrational analysis by ab initio molecular orbital calculations. Absorption bands at 2150-2300 cm -1 are assigned to the Si-H vibrations due to OSiH 3 , O 2 SiH 2 , and O 3 SiH in order of increasing frequency, respectively. The presence of H 2 O causes Si-Si bond breaking with dissociative adsorption of H 2 O, resulting in an increase in the amount of Si-H species, which is readily oxidized. The bond breaking causes a prominent enhancement of O 3 SiH in the oxidized states. This contrasts with the behavior of the oxidation in dry air where the introduction of oxygen keeps the amount of Si-H species constant in the course of the oxidation and the growth of O 2 SiH 2 and OSiH 3 in addition to O 3 SiH