Annealing and outgassing of thin films of hydrogenated amorphous silicon (a-Si:H) are shown to produce major structural changes in the material. Outgassing can occur in three stages with thresholds at 350 degrees C, 450 degrees C and 575 degrees C corresponding to conversion of SiH3 groups to SiH2 and SiH; conversion of SiH2 groups to Si and SiH; and conversion of SiH to Si respectively. Heating to 575 degrees C also appears to remove most of the hydrogen from vacancies and defects in the material. Such heat treatments could he useful for improving the stability of thin film a-Si:H devices.