The adsorption mechanism of CH3Cl on Si(1 0 0)-2 × 1 has been studied by the experiment and semiempirical calculations. For adsorption at room temperature, the existence of a precursor state was confirmed from the dependence of the sticking probability of CH3Cl on the surface coverage and substrate temperature. From Auger electron spectroscopy measurements, we have found the difference in activation barrier heights for chemisorption and desorption from the precursor state, (Ed - Ea), to be 0.28 ± 0.01 eV, indicating a nonactivated adsorption process. The optimized precursor state and the potential energy curve for the adsorption of CH3Cl on Si(1 0 0)-2 × 1 were also calculated by the semiempirical PM3 method based on a cluster model. ？ 2001 Elsevier Science B.V.