The reaction of atomic hydrogen with a growing amorphous hydrogenated silicon film plays a crucial role in determining the final material properties: hydrogen saturates dangling bonds in the film and thereby lowers the defect density, and it is assumed that hydrogen is inserted into strained bonds of the silicon network thereby reducing the local disorder. The latter reaction can be inferred indirectly since the total hydrogen uptake exceeds the decrease in the defect density. This letter presents the first direct experimental evidence fromin situinfrared spectroscopy for the insertion of hydrogen into strained Si–Si bonds. This reaction becomes visible during the addition of atomic hydrogen to an as-grown film by the appearance of a characteristic SiH vibrational mode at 2033cm−1, indicating a different embedding Si matrix compared to the standard SiH bulk vibration at 2000 cm−1. ©1997 American Institute of Physics.