Certain non-crystalline germanium films (> 10 μm in thickness) prepared by rf-sputtering crystallize “explosively” at room temperature when initiated by pricking the surface with a sharp point (or certain other methods). The propagation velocity of the crystallization at room temperature was found to be as fast a 1200 mm sec−1 depending somewhat on the conditions of film preparation, thickness, etc. The density of several such crystallizable films was determined as 5.05 g cm−3±1%. The crystallite size in the crystallized films, measured by X-ray broadening, was typically larger than 500 Å. A model for the crystallization process invokes a cascade energy transfer process, basically thermal in nature. © 1973, Chapman and Hall Ltd. All rights reserved.