The effect of silicon oxide thickness on the direct synthesis of dimethyldichlorosilane ((CH3)(2)SiCl2, dmd) was studied by reacting Si(100) surfaces with CH3Cl. Oxide layers of 0.9, 2, 4, and 14 nm average thicknesses were grown on Si(100) and were characterized by ellipsometry and AES. A copper catalyst (82 wt% Cu and 18 wt% Cu2O) was placed as a powder on the oxide layer and the reaction was carried out at 598 K in 1 atm CH3Cl. Reacted surfaces were characterized by XRD, SEM, and optical microscopy. The reacted surfaces contained Cu3Si, Cu, and Si. The surface with the lowest oxide coverage had the best selectivity (78 mol% (CH3)(2)SiCl2 and 22 mol% CH3SiCl3). As the oxide thickness increased, the selectivity for (CH3)(2)SiCl2 continuously decreased, the overall reaction rate for methylchlorosilane formation decreased, and the induction time before dmd formation increased. Even after 18 h of reaction, the rate and selectivity were still affected by the initial oxide thickness on the 2-nm oxide surface, but selectivity for dmd on the 4-nm oxide surface was the same as that on the 2-nm oxide surface. The reaction rate on Si(100) with a 14-nm oxide layer was less than 1% of that on the Si(100) with 0.9-nm oxide. On all surfaces, square-based pyramidal pits formed as Si were removed, and the pits were larger, for the same reaction time, for surfaces with thinner oxides. A 4-nm oxide affected the orientation of the Cu3Si phase, but on 0.9 and 2-nm oxide layers, Cu3Si was randomly oriented.