In the CH3SiCl3-H-2 CVD system, from which SiC-based films are prepared, the supersaturation of the gas phase increases when temperature and total pressure decreases and when a diffusion-controlled kinetic process is changed in a reaction-controlled one. These conditions variations seem to induce a transition from a growth regime to a nucleation regime, as evidenced by a study of the initial stages of the deposition. A transition from a crystallized film with;columnar crystals to a nanocrystalline deposit is also reported on the basis of accurate experiments using TEM and related techniques.