The thermal stability of the Schottky barrier height Phi(b) Of Cr-Ni-CO alloy Schottky contacts on an MBE n-GaAs substrate has been investigated using current-voltage (I-V) and capacitance-voltage (C (-2)-V) techniques after thermal annealing for 5 min in an N-2 atmosphere at several temperatures in the 200-600 degrees range. It has been found that the value of Phi(b) (0.83 or 0.84 eV) remains constant up to 600 degrees C in the forward I-V mode. Because of the presence of an interfacial layer between the alloy contact and GaAs, an ideality factor value of 1.31 was obtained for as-deposited samples. This value decreases to 1.18 with increasing annealing temperature up to 400 degrees C. At annealing temperatures above 400 degrees C, the ideality factor n starts to increase. This has been explained in terms of the presence of different metallic-like phases produced by chemical reactions between the alloy and GaAs because of the annealing process. The Phi(b)(C-V) values obtained from the reverse-bias C-2-V curves of the as-deposited and annealed diodes at 1 MHz are in the range 0.96-1.09 eV. The difference (0.13-0.26 eV) between Phi(b)(C-V) and the Phi(b)(l-V) is in close agreement with values reported in literature.