A chemisorption approach based on the Langmuir model is proposed to describe the composition of the adsorbed layer occurring on amorphous SiO2 or beta-SiC substrates in the chemical vapour deposition of silicon carbide from the Si-C-H-Cl system. The calculations of various coverages are based on (i) the calculations of partial pressures under both homogeneous and heterogeneous equilibria hypotheses, and (ii) the estimation of the adsorption equilibrium constants. H radicals, and to a lesser extent Cl, are found the most important adsorbates. The dominant source adsorbates are SiCl3, Si and C2H2, whose coverages depend on the experimental conditions and are lower on SiO2 than beta-SiC A marked inhibition effect and a selectivity of SiC deposition with respect to the substrate are pointed out and confirmed by nucleation experiments. A model for the surface chemical mechanisms is then proposed on the basis of the above chemisorption calculations and nucleation experiments.