Carbon nitride (CNx) and boron carbon nitride (BCxNy) films were deposited on silicon substrates by ion beam assisted deposition (IBAD). The films were deposited by evaporating graphite and boron carbide (B4C) targets to produce CNx and BCN films, respectively. The assistance was performed with ions from a precursor gas mixture of nitrogen and methane. IBAD has permitted to cover a wide range of compositions as a function of deposition parameters. The film growth rate presents a voltage threshold for the CNx films and a current threshold for the BCN films. The stoichiometry of the BCN films seems to be confined to two regions, depending of whether methane is included or not in the gas feed. The film composition turns towards the more stable arrangement, i.e. BN and B4C, as the gas mixture is enriched in nitrogen or methane, respectively.