The adsorption of pyrrole, aniline, 3-pyrroline, and pyrrolidine on the Si(001)-(2 x 1) surface has been studied using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Both pyrrole and aniline retain their aromatic character after bonding to the surface. Spectroscopic evidence indicates that each of these aromatic molecules can attach to the Si(001) surface via cleavage of one N-H bond, linking the molecule to the surface through a Si-N tether. Isotopic studies of pyrrole show evidence for additional cleavage of C-H bonds. While strong selectivity favoring bonding through the nitrogen atom is observed for the aromatic molecules, the unsaturated molecule 3-pyrroline shows evidence for at least two bonding configurations. XPS and FTIR data show that 3-pyrroline can bond either through the nitrogen atom with cleavage of an N-H bond, or through the C=C bond via the surface equivalent of a [2 + 2] cycloaddition reaction. Pyrrolidine appears to bond only through the nitrogen atom. Potential factors controlling the selectivity in bonding and the role of aromaticity in controlling reaction pathways on silicon surfaces are discussed.