Recently it has been proposed that alkylsiloxane self-assembled monolayers on oxidized Si(100) decompose through C-C bond cleavage [G. J. Kluth, M. M. Sung, and R. Maboudian, Langmuir 13, 3775 (1991)]. To verify this desorption picture, pentadecyltrichlorosilane precursors with four deuterated carbon atoms at the end of the molecule have been synthesized. High-resolution electron energy-loss spectroscopy shows that the monolayers are stable in vacuum up to 750 K. Above 750 K the C-D stretch disappears, while the C-H stretch remains, indicating that the end of the chain desorbs before the entire chain. From these observations it is concluded that the chains decompose primarily through C-C bond cleavage, resulting in the desorption of hydrocarbon fragments and a reduction in chain length.