The X-ray scattering of partially transformed 3C-SiC single crystals is considered in details. Extended diffuse scattering streaks, originating from stacking faults (SFs) lying in the {111} planes, are clearly observed in the wide-range reciprocal space maps. The intensity distribution along the diffuse streaks is simulated with a model including the contributions of the diffuse scattering originating from the SFs [based on the pioneering theoretical description given by Kabra et al. (J. Mater. Sci. 21, 1654-1666 (1986))], the coherent scattering emanating from untransformed areas of the crystals, as well as all θ-dependent terms that affect the scattered intensity (the layer structure factor, the irradiated volume and the polarization of the beam). The quantitative simulation of the diffuse streaks reveals that the transformation occurs through the glide of partial dislocations and allows to derive the transformation level. It is shown that the 3C polytype is indeed unstable at high temperature. However, it is further shown that defect-free 3C-SiC single crystals remain stable at temperatures where it is known to be usually unstable (1900°C). The origin of this apparent stability is very likely of kinetic nature, i.e. the lack of crystalline defects inhibits the transformation.