The interaction of 1,5-cyclooctadiene with the (001) surface of silicon has been investigated using scanning tunneling microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and ab initio computational chemistry techniques. Scanning tunneling microscopy images reveal that 1,5-cyclooctadiene molecules adsorb onto well-defined lattice sites and that the molecules are highly ordered both translationally and rotationally. Fourier-transform infrared and X-ray photoelectron spectra show that bonding occurs without dissociation by breaking one pi bond of the 1,5-cyclooctadiene molecule and the rr bond of one Si=Si dimer of the reconstructed Si(001) surface, forming two new Si-C bonds. Only one of the two unsaturated bonds in each 1,5-cyclooctadiene molecule react, leaving one double bond exposed at the outermost surface. Computational chemistry studies suggest that the surface-bound molecule involves rapid interconversions of several similar low-symmetry conformations.