Light-induced changes in vibrational absorption of SiHn have been studied for a-Si:H films. The stretching mode absorption near 2000 cm−1 decreases after light exposure. The magnitude of the change increases as the substrate temperature is lowered, and little change is observed for a-Si:H films deposited above 200°C. From the changes both for the peak and the spectrum, it is suggested that the hydrogen in the bulk diffuses to the microvoids, where it can combine to form H2 in low substrate temperature sample. The B-H modes increase after light soaking, Which is related with the increase in doping efficiency of boron, which is confirmed from the conductivity change with time during illumination.