The electronic structure and bonding geometry of acetylene adsorbed at room temperature on Si(111)-7 x 7 is studied by a combination of synchrotron radiation x-ray photoemission spectroscopy and of near-edge x-ray absorption fine structure spectroscopy. Then the stability of the molecule, submitted to thermal annealings and to synchrotron white beam irradiation is examined. The possibility of using acetylene gas as a carbon source for the fabrication of silicon-carbon compounds (or for the formation of abrupt carbon/silicon interfaces) is discussed.