We report on the thermal stability of the optical band gap and structural disorder in hot-wire-deposited a-Si:H with different atomic hydrogen concentrations. Furthermore, the changes in the structural disorder will be correlated with the changes in the optical band gap. Raman spectroscopy shows evidence that no crystallization is induced at temperatures below 550 A degrees C and that the structural disorder increases upon annealing. The increase in the structural disorder results in a broadening of the valence and conduction band tails, thereby pinning the valence and conduction band edges closer together, resulting in a decrease in the optical band gap. Hydrogen evolution occurs at 400 A degrees C via the release of molecular hydrogen trapped in voids, which results in the blistering of the sample surface.