A kinetic study was carried out on the growth of an SiC buffer layer on Si(100) by reactive chemical vapour deposition. Experiments were performed at temperatures in the range 1150-1300 degrees C for 1 to 45 min using C3H8 and H-2 as gas reactants. Infrared transmittance spectrometry was used for accurate film thickness determination (down to 1.2 nm). The growth profiles as a function of time show a four-step mechanism involving the rapid formation of an SiC "thermal layer" by coalescence of SiC islands. The thickness increases by Si out-diffusion through this layer until a critical thickness, controlled by the temperature and Si etching, is reached. Only the initial values of the diffusion profiles can be fitted by Fick’s second law. The hypothesis of silicon etching by H-2 is confirmed by thermodynamic calculations. The etching activation energy is E(e) = 4.4 eV. The temperature dependence of the resulting diffusion coefficients gives an apparent diffusion activation energy of E(d) = 4.5 eV. The close agreement between these two activation energies illustrates the competition between the two mechanisms deduced from the growth profiles.