The (001) surface of silicon contains pairs of atoms that are held together with a strong sigma bond and a weak pi bond. The interaction of styrene with the Si(001) surface has been investigated as a model system for understanding the interaction of conjugated pi-electron systems to pi-bonded semiconductor surfaces. Scanning tunneling microscopy images show one primary bonding configuration, slightly off-center from the middle of a dimer row. Infrared spectra using isotopically labeled styrene establish that attachment occurs in a highly selective way, bonding through the external vinyl group and leaving the aromatic ring almost completely unperturbed. Ab initio calculations reveal that the interaction between the pi electrons of the vinyl group of styrene and the electron-deficient end of a Si=Si dimer is strongly attractive. It is proposed that this attraction facilitates a low-symmetry interaction between the surface dimers and the vinyl group, leading to a highly selective reaction pathway for which Woodward-Hoffmann rules do not apply. The implications for selective attachment of other conjugated pi-electron systems to other pi-bonded semiconductor surfaces are discussed.