Contact reactions in the temperature range 250-650 °C between (100) Si and Pd-W binary alloy films of composition Pd80W20 and Pd30W70 have been studied by a combination of ion backscattering, x-ray diffraction, and current-voltage measurement of Schottky barrier height. For the Pd-rich alloy, the reaction around 400 °C produced the silicide Pd2Si by depleting Pd from the alloy and resulted in the formation of a two-layer structure, W/Pd2Si/Si. We have found that the W layer has served effectively as a diffusion barrier for the subsequently deposited Al, indicating that a rectifying contact and its diffusion barrier can be fabricated simultaneously. At higher reaction temperatures, the W layer transforms to WSi2 with some mixture of Pd2Si. The alloying of Pd with W has been found to increase the formation temperature of Pd2Si but decrease that of WSi2. In the Pd 80W20 reaction, Pd2Si forms around 400 °C and WSi2 around 500 °C. In the Pd30W70 reaction, Pd2Si forms around 500 °C and WSi2 around 650 °C.