The structure formation process of nanocrystalline Re-Si thin film composites with chemical compositions ReSix, x= 1.4.. 2.1 at a constant temperature slightly higher than the crystallization temperature was investigated by high-temperature X-ray diffraction and transmission electron microscopy as a function of the annealing time. It is characterized by a decreasing average crystallite size which amounts to between 7 nm and 19 nm. The formation rate of nuclei decreases as a function of (t-t(inc))(-(m-1)) where t is the annealing time at constant temperature, t(inc) is the incubation time and 0.1 less than or equal to (m-1) less than or equal to 0.6. An unknown mechanism limits the grain growth after reaching a maximum size, which decreases with increasing Si content. In the final state after annealing for more than 7000min, the films contain between 35vol% and 57 vol% of nanocrystalline phase embedded in the residual amorphous matrix.