The effect of tin on the thermal chemistry of CH3 on Si/Cu(100) was investigated using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and high-resolution helium atom scattering (HAS). The Si/Cu(100) substrate was prepared by exposure to silane at 420 K. The TPD results showed that (CH3)(3)SiH was the dominant product desorbing from Si/Cu(100) after exposure to CH3 radical, and that its desorption was sensitive to the concentration of Si in the outermost surface. For the silane-saturated surface, Si-sat/Cu(100), (CH3)(3)SiH desorbed near 500 K, while for subsaturated surfaces, Si-unsat/Cu(100), (CH3)(3)SiH desorbed at both 270 and 500 K. The addition of Sri to the Si-sat/Cu(100) surface, prior to CH3 exposure, led to a shift in the (CH3)(3)SiH desorption feature to higher temperatures. Tin addition to Si-unsat/Cu(100) surface changed the product distribution. Specifically, with Sri coadsorption levels in excess of 0.7 ML the dominant product is the fully methylated species, (CH3)(4)Si, at 430 K.