Time evolution of Si dangling bonds (dbs) was monitored during atomic hydrogen treatment of a-Si:H films using an in-situ electron-spin-resonance (ESR) technique. A high diffusion coefficient (>10−10 cm2s−1) of free atomic H in a-Si:H was detected at the very initial stage of H exposure. Atomic H diffuses into the bulk of the film (∼100 nm) and creates additional metastable dbs. The spatial distribution of such metastable dbs becomes deeper at lower treatment temperatures. An activated type of db creation reaction determines the distribution of these dbs.