Interfacial reactions of a Ni60Ta4 0 alloy film deposited on (001) GaAs substrate were studied and compared to the reactions between GaAs and single or double layer metals: Ni, Ta, Ni/Ta, and Ta/Ni. The reaction of Ni films started at a temperature as low as 100 °C by formation of Ni2GaAs and, following annealing in vacuum at 600 °C, resulted in an epitaxial β'-NiGa. On the contrary, the Ta/GaAs reaction started only at 650 °C, yielding TaAs compound. The above difference in reaction temperatures was explained by differing reaction mechanism, namely dominant diffusion of Ni atoms in Ni/GaAs reaction versus dominant diffusion of Ga and As atoms in Ta/GaAs reaction. The as-deposited alloy film was found to be amorphous and its interaction with the substrate started only at 450 °C. The reaction resulted in a phase separation where Ni diffused toward the GaAs substrate forming Ni2GaAs at the early stages of reaction, and epitaxial β'-NiGa at a later stage. The depleted Ta reacted with the out-diffusing As, forming TaAs. The reacted bilayer metal structures exhibited similar phase separation, where Ni was accumulated adjacent to the substrate and Ta in the outer layer. The presence of Ni phases at the interface with GaAs was correlated with the variation of the Schottky barrier height.