We investigated adsorption and thermal decomposition of methanethiol (CH3SH) and ethanethiol (C2H5SH) on a Si(100) surface by means of temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) with synchrotron radiation. At an adsorption temperature of 115 K, CH3SH and C2H5SH dissociate to form thiolates and hydrogen at a small coverage (<0.2 monolayer), whereas molecular chemisorption occurs at a greater coverage; all chemisorbed molecules either deprotonate to form thiolate or desorb intact up to 400 K. Adsorption and decomposition of thiols occur on the dangling bonds of a dimer without breaking the Si-Si dimer bond, resulting in preservation of a 2 x 1 LEED pattern. Thiolates further decompose to evolve hydrocarbons via scission of the C-S bond to form a sulfur adatom on the surface at a temperature above 550 K. Maximum desorption of surface sulfur as SiS occurs at 820 K. CH3 generated from CH3S reacts with surface hydrogen to evolve CH4, whereas the C2H5 moiety of C2H5S undergoes beta-hydride elimination to form C2H4. To a small extent, the alkyl moiety transfers onto the surface and undergoes dehydrogenation, resulting in desorption of hydrogen and deposition of carbon on the surface.