Microstructure analysis and contact resistance measurements of alloyed AuNiGe contacts to GaAs were performed to assist in the development of low resistance Ohmic contacts for metal–semiconductor field-effect transistor (MESFET) devices. The contact metals were prepared by sequential deposition of 100 nm of Au–27 at. % Ge, 35 nm Ni, and 50 nm Au onto sputter-cleaned GaAs wafers in which conducting channels were formed by Si doping to a level of about 1×1018 cm−3. The contact resistances were determined by the transmission line method. Analysis of the substrate and the film microstructure was carried out by x-ray diffraction, Auger electron spectroscopy (AES), and x-ray photoelectron spectroscopy (XPS). A strong correlation between the contact resistance and the film microstructure was observed. Low resistances were observed when NiAs compounds containing Ge were in contact with GaAs and the β-AuGa phase was concentrated near the top of the contact. High resistances were measured when free Au, the α-AuGa phase, or a NiGe compound were present. The temperature dependence of the contact resistance and the kinetics of compound formation were found to be influenced by the deposition sequence. Deposition of 5 nm Ni as a first layer significantly enhanced the formation of the NiAs compounds containing Ge, resulting in low contact resistance at a lower alloying temperature. However, a further increase in the thickness of the first Ni layer to 10 nm increased the contact resistance as well as the spread in the resistance values. The present study emphasizes the importance of the distribution of compounds which directly contact the GaAs in determining the contact resistance and its spread. The contact resistance increase observed previously during thermal annealing at 400 °C, after the formation of the contact, is believed to be due to the increase in contact area between GaAs and AuGa phases as a result of grain growth.