Monolayers of a series of l-alkynes, from 1-dodecyne to 1-octadecyne, have been prepared on the hydrogen-terminated Si(100) Surface via a thermal reaction of the organic compound with this Si surface. An efficient procedure is presented for the synthesis of l-alkynes from the corresponding l-alkenes. The resulting monolayers were characterized by water contact angle measurements, ATR infrared spectroscopy, and X-ray reflectivity. The results show that these l-alkynes give well-ordered, covalently bonded monolayers, which are at least as ordered as those of the corresponding l-alkenes. The exact binding geometry of the I-alkyne to the Si surface was investigated. The results from IR spectroscopy and X-ray reflectivity measurements indicate that the I-alkynes form two Si-C bonds to the surface per reacting molecule. Quantum mechanical calculations confirm that this formation of two Si-C bonds is not only chemically possible but also energetically much more favorable than formation of only one Si-C bond per reacting molecule.