The authors investigate the formation of Ge nanocrystals by thermal annealing of substoichiometric Ge O _x films fabricated by electron-beam evaporation. At the same time, they also monitor the evolution of the Ge O _x matrix. The phase separation into semiconductor and oxide phases and the evolution of Ge nanocrystals were monitored by a combination of x-ray diffraction (XRD), Raman, and transmission electron microscopy (TEM) measurements. TEM shows spherical particles of sizes in the range of 2–9 nm . They infer that an annealing temperature of 500 ° C is sufficient to generate a reasonable density of Ge nanocrystals in an amorphous Ge O _x matrix. Both XRD and Raman measurements suggest a simultaneous crystallization of the matrix at an annealing temperature of 600 ° C .