Studies of the adsorption and thermal chemistry of ethylene on the Ge(100)-2X1 surface have been performed. The results of multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy and temperature programmed desorption (TPD) show that ethylene chemisorbs molecularly on the Ge(100)-(2X1) surface at room temperature. Infrared spectroscopy gives evidence for only one adsorbate configuration on this surface at room temperature, consistent with a structure in which ethylene bridges across a germanium dimer. However, TPD measurements show two molecular desorption features at almost all coverages, indicating that at least two adsorption states can be formed. Further shifts in one of the peaks with both coverage and time, paralleled by changes in the vibrational spectrum, suggest the presence of attractive intermolecular interactions or cooperative effects. The complex time- and coverage dependence of ethylene adsorption on Ge(100)-2X1 is analyzed using a two-state kinetic model.