The chemical binding of pyridine on Si(100) has been studied using thermal desorption spectroscopy (TDS), X-ray phototelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and DFT theoretical calculations. XPS results show two chemisorption states of pyridine with N 1s binding energies at 398.8 and 401.8 eV, attributable to the [4+2]-like cycloadduct with two sigma-linkages of Si-N-1 and Si-C-4, and the dative-bonded pyridine through the lone pair electrons of its N atom, respectively. These observations were further confirmed in our vibrational studies. The formation of a dative bond between pyridine and Si(100) demonstrates a new approach for the chemical attachment of unsaturated organic molecules on the Si surface. The 1,4-dihydropyridine-like cycloadduct formed at 350 K can be considered as a template for further modification and functionalization of Si surfaces or as an intermediate for syntheses in a vacuum.