The direct process for synthesizing methylchlorosilanes from methyl chloride + silicon in the presence of catalytic amounts of copper has been studied in vacuum using a sample of Cu3Si alloy, the bulk phase that is present in active regions of the catalytic direct process. From the melt containing 23% of excess silicon, a two-phase Cu-3 Si + Si Sample was prepared. Free silicon phase served both to replenish silicon reacted from Cu3Si and to provide the grain boundaries found in an industrial process. Atomically clean surfaces of this material with varying Cu/Si atomic ratios were prepared by ion bombardment over a range of temperatures. While the dissociative adsorption of CH3Cl was observed to be immeasurably slow, on these surfaces under ultrahigh vacuum conditions, methyl + chlorine monolayers generated by the coadsorption of methyl radicals and Cl-2 led to selective formation of dimethyldichlorosilane. Adsorption of methyl groups alone produced trimethylsilane from two different active sites with very different kinetics. Adsorption of chlorine alone produced SiCl4.