IR absorption spectroscopy was used to study light-induced structural changes in hydrogenated amorphous silicon (a-Si:H) films. Our results suggest that illumination causes migration of H atoms from the interior of the film towards the illuminated surface. As a consequence, a transformation occurs in the bulk of the material leading to the formation of dangling bonds in the i-layer which could act as traps for minority carriers in solar cells. Using these results, we have formulated a model for the photodegradation of a-Si:H alloys.