Single crystal Si(100) surfaces with a native oxide layer were reacted with methyl chloride to investigate the direct synthesis of dimethyldichlorosilane. These high purity silicon surfaces are excellent models of the reacting powders us ed industrially for direct synthesis. The oxide layer did not appear to inhibit reaction significantly. The copper catalyst was added to the surface by various methods, and the form of the catalyst necessary for selective reaction was determined. Reaction was carried out at atmospheric pressure in a recirculating batch reactor and the copper-silicon surfaces were characterized before and after reaction by XRD, SEM, EDS, AES, and optical microscopy. Catalysts that contained only metallic Cu or only Cu2O did not catalyze dimethyldichlorosilane formation; both Cu and Cu2O were needed. A mixture containing 82 wt% Cu and 18 Wt% Cu2O yielded the best selectivity (65 mol% (CH3)2SiCl2, 33 mol% CH3SiCl3, and 2 mol% (CH3)3SiCl). This selectivity is comparable to those obtained in fluidized bed reactors for copper-silicon powders without promoters. Both CuCl and Cu(HCOO)2 . 2H2O catalysts were also selective for dimethyldichlorosilane formation initially, but methyldichlorosilane formed at longer reaction times. Copper formate dihydrate solution decomposed to form Cu/Cu2O mixtures on Si(100). Because the Cu percentage was lower than 82%, however, and because formate decomposition also formed a Cu film on the surface, the selectivity was lower. Most of the catalysts reacted with silicon to form the stoichiometric alloy Cu3Si, but some of the resulting surfaces did not react to form methylchlorosilanes. The competition between Cu3Si formation and consumption to form methylchlorosilanes was different for the different catalysts. A correlation was seen between epitaxial growth of Cu3Si on Si(100) and poor selectivity for dimethyldichlorosilane formation. The most selective surfaces had a randomly oriented Cu3Si phase. The Si(100) surface reacted by forming square pyramidal pits with Si(111) sides; the pits contained Cu3Si.