The electronic structure of ethylene adsorbed on Ni(110) has been studied by ARUPS using linearly polarized synchrotron radiation, and by LCGTO-LDF model cluster calculations. The adsorption system has also been characterized by TPD and LEED. The ARUPS measurement were performed for a dilute ethylene layer, where lateral interactions are not important. From the detailed analysis of the polarization, polar angle and azimuthal dependence of the ARUPS spectra we deduce an orientation of the ethylene molecules with the molecular plane coplanar to the surface and the C-C axis preferentially aligned along the [1-10] azimuth. The symmetry of the adsorption complex is determined as C1. The ethylene pi-orbital 1b2u exhibits a differential shift of 1.1 eV to higher binding energy as compared to the free molecule. The corresponding theoretical value is found to be ~ 1.7 eV. In the cluster calculations a partial optimization of the adsorbate geometry has been carried out assuming C2 symmetry. The adsorption bonding is found to be quite similar in pi- and di-sigma coordination, for both cases the pi-donation to the substrate being stronger than the pi*-backdonation. The pi-donation is mediated mainly by Nis and p contributions, pi*-backdonation mostly by Ni d orbitals. The optimized geometry parameters for the pi-bonded species are: C---C: 1.42 Å: Ni---C: 2.01 Å tilting of a CH2 group relative to the (110) crystal plane: 23°.