The interaction of hot filament “activated” methane and hydrogen with Si(100) surfaces has been probed in situ for the first time, using Auger electron and thermal desorption spectroscopies. It is shown that hot filament activation of methane results in thechemisorption of acetylene and ethylene on Si (100). Atomic hydrogen drives a range of surface reactions within the adsorbed phases formed. In particular hydrogen abstraction is observed which results in the efficient conversion of ethylene to adsorbed acetylene, and further reactions take place between atomic hydrogen, acetylene and C1 species, resulting in the formation of C3 hydrocarbon species. The implications of this work for diamond film growth are considered, and die results are used to develop a model to describe die growth of sp3 hybridised hydrocarbon chains on the Si surface.