The emission spectra of various hydrocarbon plasmas used in the growth of amorphous hydrogenated carbon (α-C:H) thin films have been measured.The presence of CH, H, H2, and C2 species in the plasma have been observed.The chemical composition of the α-C:H films was determined by combustion analysis and shows roughly equal atomic concentrations of H and C and a surprising large concentration of 0.The plasma emission spectra and chemical composition results can be partially understood in terms of relative bond strengths of the different molecules involved, and these results give some insights into the growth kinetics of α-C:H thin films.