The thermal crystallization of heavily hydrogenated, amorphous Ge (alpha-Ge:H) and ultra-pure amorphous Ge (alpha-Ge) thin films was investigated, on a comparative basis, by X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) performed during in situ heating. It was found that the crystallization process in alpha-Ge:H is pronouncedly retarded, due to the initial presence of hydrogen-induced nanovoids. The crystallization of alpha-Ge:H initiates only after considerable reduction of the amount of nanovoids in the amorphous matrix by thermally induced coarsening of the nanovoids leading to nanopores in the alpha-Ge:H. Such retarded crystallization leads to the formation of nanocrystalline, porous Ge with nanocrystal sizes below 20 nm, in contrast with the fast formation of very large-grained (grain sizes of hundreds of nanometer) polycrystalline Ge upon thermal crystallization of pure alpha-Ge. (C) 2016 Elsevier B.V. All rights reserved.