The solid state reaction between Ni-Cu and Cu-Ni bilayers on Si(100) has been studied using X-ray diffraction, transmission electron microscopy, Auger electron spectroscopy and sheet resistance measurements. The bilayers were produced by evaporation and annealed at temperatures between 200 and 500 degrees C. In the Ni-Cu sequence, a strong intermixing of silicides and a 33% reduction in the formation temperature of the NiSi2 phase compared with Ni-Si(100) was observed. When nickel was in direct contact with the silicon substrate limited intermixing and reaction of Cu was observed in the Cu-NiSi and NiSi-Si interfaces at 500 degrees C, due to the large grain size of the nickel silicide.