This Letter demonstrates that there is a good correlation between the thermal equilibrium temperatureTEand the optical gapEoptfor undoped hydrogenated amorphous silicon (a-Si: H) and a-Si-based alloys:TE, is proportional toEoptfor both wide-bandgap a-Si-based alloys, such as a-Si1–xCx: H and a-Si1–xNx: H, and narrow-bandgap a-Si1–xCx: H alloys. It is also found that the relaxation process of all thermally-induced metastable changes in these alloys follows a stretched exponential form.