The adsorption and reaction of acetylene dosed on the Si(111)-(7 x 7) surface has been studied by means of X-ray photoemission, Auger, and HREELS spectroscopies for different substrate temperatures. For exposure at temperature of 600 degreesC, only an extremely thin reacted layer can be obtained, which hinders further molecular reaction at the surface, acting as a barrier layer. The increase in surface diffusion occurring between 600 and 750 degreesC overrides the barrier layer effect, allowing the growth of a thicker SiC.