Structural processes occurring in thin Au(Zn) metallization during the formation of ohmic contact to p-type GaAs have been investigated by X-rav diffraction. secondary-ion mass spectrometry, transmission electron microscopy and high resolution electron microscopy. In order to examine the release of arsenic during thermal processing, a thin film collector method was applied. Emphasis was placed on the particular role of each of metallization constituents during consecutive stages of the formation of an ohmic contact. Pure Zn was found to penetrate the native oxide on GaAs surface and to form an ohmic contact after annealing at 220-degrees-C. The addition of Zn into the Au metallization suppresses the thermally induced decomposition of GaAs and stabilizes the size of Au grains, forming the alpha3-AuZn phase. The most important aspect of the GaAs Au reaction is the formation of Ga vacancies in the subcontact region. They are required to form a low resistance ohmic contact to p-type GaAs. since the specific resistance of the Au(Zn) contact is one order of magnitude lower than that of the pure Zn contact.