Ab initio QCISD(T)/6-311 + + (2d,2p) calculations have been carried out for an extensive study of gas-phase reactions among CH4, NH3 and their radicals. Our study shows that stable HCN molecules are readily formed by successive H abstraction reactions. Some of the reactions are strongly exothermic and have negligible energy barriers. In agreement with some recent experiments, our results indicate that H abstraction reactions, which make the chemical vapor deposition of diamond thin films successful, do not favor the formation of carbon nitride thin films.