The present invention is directed to a method of producing diamond films through the thermal dissociation of molecular chlorine into atomic chlorine in a heated graphite heat exchanger at temperatures of from about 1,100.degree. C. to about 1,800.degree. C. The atomic chlorine is subsequently rapidly mixed with molecular hydrogen and carbon-containing species downstream. Atomic hydrogen and the carbon precursors are produced through rapid hydrogen abstraction reactions of atomic chlorine with molecular hydrogen and hydrocarbons at the point where they mix. The mixed gases then flow across a heated substrate, where diamond is deposited as a film. Diamond deposits have been confirmed by Raman spectroscopy.