Interfacial reactions between ultra-thin Ni/Co bilayer films and single crystal Si (100) substrate were studied at 600 degrees C using X-ray photoelectron spectroscopy (XPS). In very thin bilayer films (<20 nm), only CoSi2 and NiSi2 were detected in the reaction layer. When the total thickness of the film was increased to 30 nm along with an increase of thickness of the Co layer with respect to the Ni layer, NiSi formed along with CoSi2 and NiSi2. The resulting silicide film also had a very low sheet resistance. The concept of effective heat of formation has been used to explain the experimentally observed reaction layer structure.