Amorphous films of Ni‐W and Ni‐N‐W were deposited on single‐crystal silicon with discharge gases of Ar or Ar+N2 by rf cosputtering of Ni and W. The reaction of these Ni‐W and Ni‐N‐W films with the Si substrate were studied in the temperature range of 450–750 °C by a combination of backscattering spectrometry, x‐ray diffraction, cross‐sectional transmission electron microscopy, and resistivity measurements. Films with composition Ni36W64 are stable below 500 °C. NiSi and NiSi2 form at 500 °C, and WSi2 forms rapidly in the temperature range of 625–650 °C. The nickel silicide forms adjacent to and within the silicon, while the outer layer becomes a mixture of WSi2 and NiSi2. The morphologies of the reacted layers are revealed by cross‐sectional transmission electron microscopy. The crystallization temperature of amorphous Ni36W64 films on SiO2 is near 650 °C also. Adding nitrogen to form amorphous Ni30N21W49 films lowers the crystallization temperature, but raises the reaction temperature with Si to 750 °C.