A fixed-bed reactor was employed to study the methylchlorosilane (MCS) reaction, also called the direct process. We used a copper-silicon contact mass as the main MCS catalyst. The copper-silicon contact mass was prepared by the reaction of CuCl with silicon. The effect of added zinc and phosphorus to the MCS reaction was explored, and we found that, at CU/Zn ratios > 30, phosphorus addition resulted in an increase in selectivity for dimethyldichlorosilane (Di) at the expense of methyltrichlorosilane (Tri) and residue. Addition of tin to the MCS reaction resulted in an increased overall rate but with a decrease in Di and an increase in disilane formation. Addition of phosphorus and high tin levels resulted in a high rate but with a high selectivity for Di; phosphorus negated the selectivity penalty caused by the addition of tin alone. Phosphorus appeared to cause an increase in formation of the eta phase (Cu3Si), as determined by analysis of MCS beds formed under different conditions. Previously reported arguments show that copper diffusion is operative for Cu-Si renewal in MCS but that Cu diffusion is not the rate-limiting step in the reaction.