Amorphous carbon films were deposited at room temperature on the anode and cathode of a radio-frequency glow discharge reactor by the dissociation of acetylene-argon mixtures. The film's properties ranged from low density, soft polymer-like to higher density a-C:H. The changes in the atomic structure, optical and electrical properties were studied as functions of the pressure. Anodic films are highly resistive, have a low refractive index (n approximate to 1.76-1.94) and a wide band gap (E-g approximate to 1.7-2.5 eV), whereas cathodic films are more conductive to (sigma approximate to 10(-12)-10(-9) S cm(-1)), have a higher refractive index (n approximate to 2.0-2.35) and a smaller band gap (E-g = 1-1.3 eV). The conductivity and activation energy of cathodic films can be controlled by the addition of phosphine or diborane to the gas mixture, suggesting that doping is possible in a-C:H as in hydrogenated amorphous silicon.