The adsorption of pyridine at room temperature on single-domain Si(100)-2 x 1 surfaces at saturation coverage has been investigated by means of fully polarization resolved near-edge X-ray adsorption fine-structure spectroscopy (NEXAFS) experiments. The major product of pyridine chemisorption has been identified as the 1,4,5,6-tetra-sigma-bonded species that is grafted on two successive dimers keeping a C-2=C-3 bond parallel to the dirtier axis.