The effects of precursor gas composition and radio frequency (r.f.) power on the optical and structural properties of a-C:H/SiC:H composite thin films are investigated. The composite films are deposited at 25 degrees C in a 13.56 MHz, r.f. driven asymmetric plasma reactor from a mixture of benzene. tetramethylsilane (TMS) and hydrogen. The structure and properties of the films are evaluated by an ellipsometer, Fourier transform infrared (FTIR) spectroscopy, a stress gauge, and an ultraviolet-visible spectrometer. The FTIR spectra of the composite films show characteristic peaks of a-C:H and a-SiC:H components. The precursor gas composition and r.f. power input are shown to affect the optical and structural properties of the composite and r.f. power input are shown to affect the optical and structural properties of the composite films significantly. As the fraction of TMS in the feed gas is increased, the deposition rate, transparency, and optical band gap of the film are increased, while the refractive index, intrinsic stress, and graphitization of the film are decreased. As the r.f. power is increased at a constant reactor pressure, the negative self-bias voltage at the substrate holder is increased and this results in decreases in the optical band gap, and increases in the intrinsic compressive stress and graphitization of the film.