Annealing behavior of Au, AuZn, and AuSb metallization on GaSb have been investigated by the combined use of RBS, XRD, TEM, and I-V characterization. The results give evidence that the thermally activated contact reaction strongly depends on the particular elements incorporated in the Au layer. Pure Au reacts with GaSb at 100°C. The addition of Zn to Au metallization increases the thermal stability of the metallization/semiconductor system to 200°C. Antimony, forming with gold the AuSb2 phase in metallization, provides the most stable ohmic contact system.