Abstract The light- and thermal-induced changes of SiH bonds in undoped a-Si : H have been studied on the infrared absorption of SiH stretching mode, using the infrared photothermal deflection spectroscopy and the photothermal bending spectroscopy. The results show that the IR absorption of SiH bonds increases by light soaking of the high power ∼ 500 mW/cm 2. The change of IR absorption of SiH bonds reoccurs by thermal annealing at 200°C. This change is related to the increase of the dangling bonds under light soaking. Furthermore, we observe the change of the elasticity modulus by light soaking, using the photothermal bending spectroscopy. The structural change in a-Si : H is discussed based on these results.