Films were deposited from glow discharge plasmas of acetylene-oxygen-argon mixtures in a deposition system fed with radio frequency power. The principal variable was the proportion of oxygen in the gas feed, X(ox). The chemical structure and elemental composition of the films were investigated by transmission infrared spectrophotometry and x-ray photoelectron spectroscopy. Optical properties-refractive index, absorption coefficient, and optical gap-were determined from transmission ultraviolet-visible spectroscopy data. The latter also allowed the determination of film thicknesses and hence deposition rates. It was found that the oxygen content of the films and, within limits, the refractive index are controllable by the selection of X(ox).