Changes in hydrogen environments in porous silicon prepared from a p-type silicon wafer with thermal annealing were studied to make an exact assignment of the infrared absorption spectrum and to clarify the fine structures of hydrogen in porous silicon. Annealing makes it possible to desorb the SiH3 and SiH2 states from porous silicon. The results, with the help of the vibrational analysis by molecular orbital calculations, revealed the following. First, an absorption band at 2100 cm(-1) is composed of seven fine structures and their assignment is clarified. Second, the wagging mode of SiH2 contributes an absorption band at 667 cm(-1), and finally it is confirmed that the strongest absorption band at 626 cm(-1) belongs to the bending vibration of SiH. Further, oxidation behavior under wet environments of porous silicon in which hydrogen is partially desorbed is compared with that without the desorption. Acceleration of the oxidation is explained in terms of strained Si-Si bonding.