The covalent attachment and binding configuration of benzonitrile on Si(100) have been studied for exploring the selective binding of multi-functional molecules on Si surfaces using temperature-programmed desorption (TPD), high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and DFT calculations (pBP/DN**). Both chemisorbed and physisorbed benzonitriles were identified at an adsorption temperature of 110 K. Chemisorbed benzonitrile desorbs molecularly at similar to490 K while physisorbed molecules desorb at similar to180 K. Vibrational features of chemisorbed benzonitrile unambiguously demonstrate that the cyano group directly interacts with S! surface dangling bonds, evidenced in the disappearance of the CdropN stretching mode around 2247 cm(-1) coupled with the appearance of the C=N stretching mode at 1629 cm(-1) and the retention of all vibrational signatures of a phenyl ring. XPS shows that both C Is and N Is BEs of the cyano group display large downshifts by 2.4 and 2.0 eV, respectively. A smaller downshift of similar to0.7 eV is oberved for phenyl group due to the weaker inductive effect of the C=N group of chemisorbed benzonitrile than that of the CdropN group of physisorbed molecules. Compared to physisorbed molecules, the contribution of photoemission from pi(CN) of chermisorbed benzonitrile is significantly reduced, confirming the direct involvement of pi(CN) in the surface binding. Experimental results show that the covalent attachment of benzonitrile on Si(100) occurs in a highly selective manner through the direct interaction of both C and N atoms of the cyano group with a Si Si dimer to form a four-membered SiCNSi ring at the interface, leaving a nearly unperturbed phenyl ring protruding into vacuum. This functionalized Si(100) with a phenyl ring may be employed as a substrate for fabricating multilayer organic thin films on Si surfaces or serve as an intermediate for chemical syntheses in a vacuum.