The dependence of Schottky barrier height (φB) derived from current-voltage measurements on annealing temperature of thin films of Ni, Ta, Ni/Ta, and Ta/Ni bilayers, and Ni60Ta4 0 alloy on n-type (100) GaAs substrate was determined. Variations in the measured φB values have been related to the appearance of different phases at the interface with GaAs resulting from chemical reactions between the metal films and the GaAs substrate. Both Ni and Ta form with n-GaAs rectifying contacts with barrier height higher for Ni than for Ta; at a certain temperature related to compound formation (550°C for Ni and 650°C for Ta) the contacts become ohmic. For the bilayers and alloyed films the presence of Ni phases (Ni2GaAs at 350°C and NiGa at 550°C at the interface with GaAs due to phase separation during annealing determines the electrical properties of the contact (rectifying up to 350°C and ohmic at 550°C). The φB measurements were proved to be very sensitive to the initial stages of interfacial reactions. The phase separation can be potentially used for formation of shallow contacts to n-GaAs.