The surface chemistry of diethylsilane (DES) and diethylgermane (DEG) on the Si(100) surface was studied using high-resolution electron-energy-loss spectroscopy and temperature programmed desorption. High-resolution electron-energy-loss spectra indicate that the precursors chemisorbed dissociatively as (C2H5)2MHx(ad) and (2-x)H(ad) [x = 0,1; M = Si or Ge] groups at room temperature and that the ethyl groups remain bonded to the precursor Si or Ge. Thermal annealing of DEG covered surfaces indicated that the ethyl groups remain attached to Ge at least up to the desorption temperature. Ethyl ligands react to form ethylene via a beta-hydride elimination pathway. The ethylene desorption peak temperature from DEG dosed surfaces was approximately 40 K lower than that from DES dosed surfaces (733 K). We propose that ethylene desorbs from Ge atoms rather than from Si surface atoms. The hydrogen remaining on the surface after ethylene desorption desorbs from the beta1 state, with peak maxima at 810 and 780 K for DES/Si(100) and DEG/Si(100), respectively.