The growth of CoSi2 has been studied by deposition and reaction of Co on clean Si (111) surfaces in situ in a modified ultrahigh vacuum transmission electron microscope. On deposition of nominally 20 Å Co at room temperature, strong interaction between Si and Co occurs yielding an epitaxial film which is believed to be a hexagonally distorted form of the Co2Si structure. On heating to ∼350°C the growth of epitaxial CoSi is observed, which transforms to CoSi2 at ∼450°C. Silicon-rich phases propagate by lateral motion of phase boundaries. Substantial pinhole density arises in films only after annealing at higher temperatures. The B orientation dominates under similar annealing conditions. In contrast to NiSi2, the growth of cobalt silicide films on Si is dominated by bulk phase formation, with interfacial energy minimized in all cases by epitaxy.