We present a systematic analysis of the conditions for the formation of Ge nanocrystals in GeOx and SiGeOx alloy films during annealing. Amorphous SiOx, GeOx and SiGeOx alloy films prepared by d.c. magnetron sputtering were subsequently annealed for times between 10 min and Ga min and at annealing temperatures ranging from 350 degrees C up to 800 degrees C. The formation nanocrystals is analysed by X-ray scattering under grazing incidence. No crystallisation is found for the Sig, films under any annealing conditions. On the other hand, clear evidence for the formation of Ge nanocrystals is found in the GeOx as well as in the SiGeOx films. For the GeOx films the particle diameter d(s) linearly decreases with increasing oxygen content yielding values of d(s) = 40 nm down to 10 nm. The as-prepared amorphous SiGeOx films were found to always consist of nanoamorphous Ge clusters in an amorphous SiOx matrix determining the size of the resulting Ge nanocrystal after annealing. A Ge nanocrystal diameter of around d(s) = 6.0 nm is found for all SiCeOx alloy films independent of the annealing time and temperature.