As-deposited and wet oxidized a-Si:B alloys deposited by low-pressure chemical vapor deposition with various boron content were studied by visible optical spectroscopy and Fourier transform infrared spectroscopy. It is found that the optical band gap of a-Si:B varies with respect to the boron content. This effect is associated with the structure change induced by B content. After oxidation, the Si–O stretching peak at 1108 cm-1 shifted to 1079 cm-1. The oxidation behaviors of films with boron content 3%–25% are different from those with boron content exceeding 30%. For B content between 3% and 25%, besides the remarkable change around 1100 cm-1, there are also notable changes around 1600–1300 cm-1, and 1000–500 cm-1, which indicate the increase of B–O bonds and decrease of B–B bonds. However, when the B content exceeded 30%, the change of Si–O bonds was enhanced while the change of B–O bonds and B–B bonds was suppressed. A possible structure and oxidation mechanism for different boron content a-Si:B films is proposed. © 1998 American Vacuum Society.