A triple sandwich of C&sngbnd;Se&sngbnd;C was employed to study the transformation behavior of amorphous selenium by electron microscopy. The electron beam was used as energy source in all experiments. Reversible transformations from crystalline to amorphous Se was observed in well‐defined localized areas. By application of a rapid heat pulse, void formation occurred in amorphous Se. Slow heating caused erasure of the voids by viscous flow. Crystalline Se held at low temperatures (< 50°C) was found to become amorphous by gradual breakdown of crystalline order under electron irradiation.