The copper-catalyzed direct synthesis of methylchlorosilanes was carried out at atmospheric pressure and 550 K on four planar Cu3Si alloy surfaces contained <0.1 at% zinc and tin promoters. Kinetics were measured in a batch reactor attached to an ultrahigh vacuum chamber that allowed Auger surface analysis after reaction. Concentration depth profiles were obtained by ion sputtering and Auger analysis. These measurements showed that reaction almost completely depleted silicon in the subsurface regions of both unpromoted and tin-promoted Cu3Si samples. The presence of zinc promoter in low concentrations (0.04-0.06 at%), however, dramatically increased silicon diffusion rates so that the subsurface regions of zinc-promoted Cu3Si samples were not depleted of silicon. The increased diffusion rate is probably due to an increased rate of grain. boundary diffusion. A synergistic interaction between zinc and tin further increased the subsurface concentrations of silicon. This dramatic increase in the silicon subsurface concentration in the presence of low concentrations of zinc indicates one important role of zinc promoters in the catalytic direct synthesis of methylchlorosilanes.