Mixtures of C2H2, He and N-2 were polymerized in an r.f. discharge. Quantitative optical emission spectroscopy was used to determine relative concentrations of the key species CH and CN in the plasma as a function of the proportions of monomer gases in the feed, and to delineate the behavior of the electron mean energy and number density in the discharge. Transmission IR spectroscopy (IRS) and electron spectroscopy for chemical analysis (ESCA) of films deposited from discharges containing different proportions of nitrogen revealed the incorporation of both CH and CN in the deposited material. Deposition rates were also determined and their connection with deposition mechanisms via gas-phase and surface reactions is outlined.