In order to elucidate the diffusion mechanism of hydrogen in post‐plasma‐hydrogenation of amorphous silicon (a‐Si) film prepared by chemical vapor deposition (CVD), the change in the hydrogen depth profiles with plasma exposure time and with successive hydrogenation of hydrogen isotopes were measured by secondary ion mass spectrometry and infrared absorption. The post‐hydrogenation process of the CVDa‐Si film is explained by a model composed of fast diffusion (small activation energy) of atomic hydrogen through weakly bound sites such as interstitials, its capture by reactive sites such as weak SiSi bonds and dangling bonds, and an exchange between weakly bound and bonded hydrogens.