The adsorption of phenyl isothiocyanate (PITC) on silicon (001) surfaces has been investigated using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, scanning tunneling microscopy (STM), and ab initio calculations. The experimental evidence shows that adsorption onto the Si surface occurs in a highly selective manner, through the N=C=S group and not through the benzene ring. Both experimental and theoretical evidence suggests that the adsorption involves the interaction of the N and C atoms of the N=C=S group with the Si=Si dimer, forming a four-member Si2NC ring at the interface. This process leaves the aromatic ring nearly unperturbed and is analogous to a 1,2-dipolar addition reaction. Theoretical calculations confirm that this is the most stable product.