The reaction mechanisms taking glace in a film-forming radio frequency discharge of a mixture of acetylene, nitrogen, and helium have been investigated using optical emission spectroscopy. A transient actinometric method was employed, based on the interruption of the flow of one of the feed gases (either C2H2 or N-2) ana the subsequent observation of the time-dependent changes in the discharge environment, specifically of the concentrations of CH and CN species, two possible precursors of film formation. Both N-2 and C2H2 affect the CH and CN concentrations. Nitrogen strongly enhances the rate of formation of CH, very probably via gas phase reactions between atomic nitrogen and C2H2 or other species containing carbon and hydrogen. On the other hand, the relatively high CH concentration observed in discharges without nitrogen is taken to indicate that fragmentation of the C2H2 molecules by the discharge is another important mechanism of formation of the CH species. For the CN species; gas phase reactions between carbon- and nitrogen-containing species. are expected to contribute to its appearance. However, a relatively high CN concentration could be detected even in discharges without C2H2 in the gas feed, so long as a nitrogen-containing polymer film was previously formed on the inner wall of the chamber. This is taken as evidence that interactions between the plasma and the polymer surface play an important role in the generation of the CN species. Further measurements of the CN concentration in a plasma of 100% He, produced in the chamber immediately after the formation of a nitrogen-containing polymer film, indicate that a possible reaction mechanism for the formation of the CN species is the detachment of nitrile groups (-C=N) from the poly;ner surface upon cleavage or the carbon bond Linking-the nitrile terminations to the polymerchains.