Abstract The adsorption and thermal behavior of methyl iodide on Si(100)-(2 × 1) have been studied by kinetic uptake measurements, Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD) mass spectroscopy. Methyl iodide adsorbs on Si(100) at room temperature with high efficiency (initial reaction probability of unity). From the kinetic uptake measurements and the AES measurements an average saturation coverage of (2.9 ± 0.3) × 10 14 methyl iodide molecules cm 2 is calculated, i.e., about one methyl iodide molecule per two silicon surface atoms. Experimental evidence is presented indicating that the molecule dissociates into a covalently bonded methyl group and an iodine atom upon adsorption. Heating causes the decomposition of the methyl group. The hydrogen atoms liberated from the methyl group mainly desorb as molecular hydrogen. The adsorbed iodine desorbs in the form of atomic iodine and possibly some small amount of hydrogen iodide as detectable by line-of-sight mass spectrometry. Less then 1% of the carbon desorbs in the form of C 2 hydrocarbon species. Neither the desorption of methane nor the desorption of methyl iodide is observed.