Actinometric optical emission spectrometry was used to determine the relative concentrations of the species H, CH, N-2 and CN as a function of the ratio of the nitrogen flow rate to the acetylene flow rate in acetylene-ni.rogen-helium-argon mixtures. The forms of these concentration functions are discussed in terms of gas-phase molecular fragmentation and plasma/film-surface interactions. In plasmas fed only noble gases, additional experiments revealed the occurrence of plasma/film-surface reactions that produce significant plasma concentrations of the species CN provided that a nitrogenated carbon film was previously deposited in the chamber. In contrast, there is no appreciable concentration of the species CH in plasmas of noble gases in the presence of a-C:H:N films on the chamber walls.