Ethylene adsorption on vicinal, single-domain Ge(100)-(2x1) has been investigated by thermal desorption spectroscopy (TPD) and angle-resolved photoemission (ARUPS) using linearly polarized synchrotron radiation. Thermal desorption experiments show that chemisorbed C2H4 desorbs from Ge(100) nondissociatively around 393 K with a high temperature shoulder which is tentatively assigned to step site desorption. The ethylene saturation coverage is strongly temperature dependent. Adsorption at 90 K saturates at 0.38 monolayer (ML), whereas adsorption at 170 K leads to a saturation coverage of approximately 1 ML. This behavior is explained by an adsorption barrier for coverages exceeding 0.38 ML. ARUP spectra for a dilute and the saturated ethylene monolayer reveal clear differences. Using photoemission selection rules a highly (C-2v) symmetric adsorption geometry with a C-C bond axis parallel to the Ge-Ge dimer axis is found for the dilute layer; whereas a reduced C-2 adsorption symmetry is found for the saturated ethylene layer. The comparison of photoemission spectra for C2H4 on Ge(100) and Si(100) shows that C2H4 is di-sigma bound to the dangling bonds of a single Ge-Ge dimer. For two molecular orbitals, 1b(3u) and 1b(2g), one-dimensional band structures with dispersion widths of 0.5 and 0.39 eV, respectively, along the Ge-Ge dimer rows are found which present a straightforward explanation for the observed symmetry reduction and adsorption behavior.