X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) studies were used to investigate the reactions of coadsorbed CH3 and Cl2 on Si/Cu(100). TPD results showed that the individual exposure of Si/Cu(100) to CH3 and Cl2 resulted primarily in the desorption of (CH3)3SiH and SiCl4, respectively. Coadsorption of CH3 and Cl2 at specific surface concentrations on Si/Cu(100) resulted in the desorption of (CH3)4−xSiClx (x ranges from 1 to 3) species. The relative surface concentration of CH3 and Cl (resulting from Cl2 dissociation), however, controlled the stoichiometry of the methylchlorosilane product. XPS results suggested that more Si was removed from the Si/Cu(100) surface as gaseous product when CH3 and Cl2 were coadsorbed on the surface than when CH3 and Cl2 were adsorbed alone.