The adsorption and decomposition of trimethylsilane, HSi(CH3)3, on Si(100) has been studied at T approximately 120 K for a variety of trimethylsilane exposures. The trimethylsilane molecule is not observed by quadrupole mass spectrometry during temperature programmed desorption from the Si(100) sample, but various fragments of trimethylsilane can be seen. The primary species seen desorbing during electron bombardment of the Si(100) surface were the H+, H-, and CH3+ ions. Both H+ and H- KEDs exhibit bimodal kinetic energy peaks, most likely due to hydrogen bound in multiple states. The bimodal H+ and H- kinetic-energy distributions (KEDs) exhibit an interesting reversal of intensities that suggests that the state giving rise to the low-energy peak in one KED gives rise to the high-energy peak in the other KED, and vice versa. Kinetic energy distributions for the CH3+ ions consisted of only one peak, indicating a high probability that the CH3+ originates from only one surface state. Our results indicate that electron-simulated desorption can be used as an adsorption state sensitive probe of chemical species on semiconductor surfaces.