The scanning tunneling microscope has been used to characterize the adsorption sites of 1,9-decadiene on Si(111) 7 x 7. The molecule which reacts with each of its two ends, appears as pairs of reacted adatoms on the atomically resolved surface. The size of the molecule permits adsorption with the two ends of the molecule situated either at the same half of the unit cell or in a bridging geometry with one end situated in the faulted half and the other in the unfaulted half. Three types of bridging pairs are found. The results found are compared with that of two other dienes : 1,7-octadiene and 1,13-tetradecadiene.