The reaction of copper with silicon in Cu-Si bilayers with overall compositions between copper and the most silicon-rich compound Cu3Si, was monitored by means of several analytical tools, in situ resistance measurements during controlled heating, backscattering, and X-ray diffraction. The order of phase formation was established to be first η″ Cu3Si, followed by the formation of a phase called γ, with about 17 at.% Si. With a sample of intermediate composition, one then observes the reaction of η″ with γ, resulting in the formation of the ε phase with about 20 at.% Si. The resistivity of the various phases was estimated. © 1991.