Benzene adsorption on a single-domain Si(001)-(2X1) surface has been studied by thermal desorption spectroscopy (TPD) and angle-resolved photoelectron spectroscopy (ARUPS) using linearly polarized synchrotron radiation. Angle-resolved photoemission spectra for the saturated benzene layer exhibit well-defined polarization and azimuthal dependencies compatible with a flat-lying benzene molecule with local C-2 upsilon symmetry. Based on these results two structure models are proposed. First-principles density functional cluster calculations have been performed for each of these structures, Total energy minimization and a detailed comparison of the experimental ARUPS spectra with the one-particle spectra of the model clusters leads to a 1,4-cyclohexadienelike adsorption complex with a flat-lying benzene molecule which is di-sigma bonded to the two dangling bonds of a single Si-Si surface dimer. Especially, one of the unoccupied 1 e(2u) (pi*) orbitals of the free benzene molecule shifts down (by about 3 eV) and evolves into the highest occupied molecular orbital (HOMO) of the chemisorbed molecule.