The thermal behavior of alkyltrichlorosilane (CH3(CH2)(n-1)SiCl3) based self-assembled monolayers on the oxidized Si(100) surface has been examined under ultrahigh vacuum conditions for n = 4, 8, and 18. Using high-resolution electron energy loss spectroscopy and contact angle analysis, it is found that the monolayers are stable in vacuum up to about 740 K independent of chain length. Above 740 K the chains bean to decompose through C-C bond cleavage, resulting in the desorption of hydrocarbon fragments. Following this initial desorption step, methyl groups attached to silicon atoms are observed. The siloxane head groups remain on the surface following decomposition of the monolayers until about 1100 K.