The gas phase during the chemical vapor deposition of silicon carbide from CH3SiCl3 has been investigated by means of FTIR spectroscopy in the in situ conditions. Results show the formation of SiCl4 and CH4 molecules which are the transition products in the deposition process, according to earlier suppositions. The gas phase reaction induced by small amounts of HCl (or H2O) in the system is an autocatalytic one. The mechanism of surface reactions is proposed. The importance of gas phase analysis in the deposition process is indicated.