Hydrogenated amorphous silicon (a-Si:H) films have been thermally annealed at temperatures in the range 220-270 °C for 24-48 h either under intense visible light illumination (4-16 W/cm2) or in the dark. After each annealing, the hydrogen-concentration profile was measured with Rutherford-backscattering-spectrometry and elastic-recoil-detection-analysis ion-beam analysis methods. A model is proposed which shows that, in good agreement with our results, the hydrogen-diffusion constant DH is proportional to the power of illumination and also proportional to the loosely bonded hydrogen concentration. Other consequences of the model are discussed. © 1994 The American Physical Society.