An in situ scanning electron microscope study of the effect of heat treatment on the structural integrity of undoped and phosphorus-doped glow discharge amorphous silicon films, deposited on crystalline silicon and pyrex 7059 glass substrates, has been undertaken. A similar, albeit more limited, investigation has been carried out on freely supported glow discharge films and one reactively sputtered Si-supported film. Glow discharge films, prepared at temperatures >70° C on silicon substrates, exhibit "blistering" of the surface at dehydrogenation temperatures, namely, >270° C. Ultimately, the "blisters" burst forming circular craters which reveal the underlying substrate. A similar phenomenon was observed in the case of a reactively sputtered film. In contrast, films prepared on silicon substrates at nominally room temperature exhibit more complex behaviour in which complete disintegration of the film occurs. Neither of the above effects was observed, on heat treatment, for amorphous silicon films which were either freely supported or deposited on pyrex glass. © 1981 Chapman and Hall Ltd.