The room-temperature adsorption and desorption kinetics of diethylsilane (DES) and diethylgermane (DEG) on the Si(100)-(2X1) surface were investigated under ultrahigh vacuum using temperature programmed desorption, high resolution electron energy loss spectroscopy, and Auger electron spectroscopy. DES and DEG adsorb at room temperature in a self-limiting fashion, reaching saturation (0.4 and 0.3 monolayers, respectively), at exposures above 30 and 350 L, respectively. Temperature programmed desorption of the DES-saturated and DEG-saturated surfaces revealed only two species, hydrogen and ethylene, desorbing from either surface. In both systems, the hydrogen atoms desorbed primarily through the recombinative desorption of monohydride species, while the ethyl groups decomposed via P-hydride elimination and subsequently desorbed as ethylene. The hydrogen desorption peak temperature was 794 K for the DES-saturated surface and 788 K for the DEG-saturated surface. The desorption peak temperature for ethylene was significantly lower in the DEG/Si(100) system (700 K) than in the DES/Si(100) system (730 K) because of a lower activation energy and higher pre-exponential factor for P-hydride elimination from DEG-dosed Si(100). High resolution electron energy loss spectra of the DEG-saturated surface support an adsorption mechanism in which the ethyl groups remain bonded to the incoming germanium atom throughout the adsorption process.