The precise C content of a series of Si1-yCy epilayer samples (0≪y≪0.012) was determined by resonant backscattering experiments using a 4He+ ion beam at 5.72 MeV. This beam energy is more suitable for the determination of the C content than the previously used 4.265 MeV. From the correlation of these investigations with x-ray diffraction experiments, a significant deviation of the lattice parameter variation in Si1-yCy from Vegard’s rule between Si and diamond or β-SiC was observed, which amounts up to 30% or 13%, respectively, for y≪0.012. This negative deviation is in agreement with recent theoretical predictions by Kelires. © 1998 American Institute of Physics.