detailed study was carried out in investigating the chemical reaction scheme involved with the deposition of a-SiC:H films by carbosilane sources in a remote hydrogen plasma. As the carbosilane sources, tetramethylsilane (TMS), hexamethyldisilane (HMDS) and tetrakis(trimethylsilyl)silane (TMSS) were used. Depositions were performed in the presence and absence of both UV radiation and H radicals. UV radiation activates only the TMS molecules out of the above three types of molecules in depositing a-SiC:H films. Other two monomers are seen to be activated by hydrogen radicals. It is proposed that the most susceptible bond to be broken at first by hydrogen radical is the Si-Si bond. A reaction model for the formation of a- SiC:H films was presented by considering the Me2Si=CH2 as the film forming precursor.