Summary. --An ion-beam-assisted deposition (IBAD) technique, based on reactive evaporation using hydrogen/methane gas mixture, has been used for the preparation of a-Sil _ x Cx: H films. Measurements are reported on the composition, optical gap, infra-red vibrational absorption bands and on the electrical darkconductivity temperature dependence to verify the reliability of this deposition method. On increasing the compositional parameter x up to 0.35, the IR results show an increasing hydrogenation and the presence of Si-CH~ units in addition to the ones with the carbon fully coordinated with the silicon, while the optical-gap and the dark-conductivity activation energies reached the values of 2.44 eV and 0.77 eV, respectively. PACS 81.15 -Methods of deposition of films and coatings. PACS 78.65 -Optical properties of thin films, surfaces, and thin layer structures (including superlattices, heterostructures, and intercalation compounds).