Densely-packed alkyl monolayers similar to those previously reported by Linford et al.(1,2) are formed by the reaction of 1-alkenes with hydrogen-terminated surfaces of both Si(lll) and Si(100). The thermal behavior of these monolayers in vacuum has been studied using high-resolution electron energy loss spectroscopy. Both on Si(lll) and on Si(100), the monolayers are found to be stable up to about 615 K. Desorption is signaled by a decrease in the intensity of C-H modes, accompanied by the appearance of Si-H modes, which suggests that desorption occurs through beta-hydride elimination reactions. Upon further annealing to 785 K, C-H and Si-H modes essentially disappear, and a peak appears at 780 cm(-1), which is attributed to a SiC vibrational mode. This behavior indicates that decomposition of the monolayers has taken place.