Contact reactions between a Si substrate and thin films of TaW bilayers and codeposited alloys have been studied. The interdiffusion and silicide formation have been analysed by Auger electron spectroscopy and X-ray diffraction. In the bilayer structures the formation of TaSi2 and WSi2 is observed at 700-750°C and 800-850°C, respectively. For the Ta-rich alloy of Ta80W20, TaSi2 is formed at 700-750°C similarly to the bilayer case, but this is followed by the formation of Ta5 Si3 which disappears at higher temperatures. For the W-rich alloy of Ta20W80, silicide formation is delayed to higher temperatures due to alloying in the thin film.