Solid-state diffusion of the transition metal impurities, gold (Au), nickel (Ni), and copper (Cu), in silicon (Si) nanowires was studied by in situ transmission electron microscopy. Compared to diffusion in a bulk crystal, Au diffusion is extremely slow when the amount of metal is limited but significantly enhanced when an unlimited supply is available. Cu and Ni diffusion leads to rapid silicide formation but slows considerably with physical encapsulation by a volume-restricting carbon shell.