Contacts consisting of tungsten, W0.79Si0.21, W0.67Si0.33 and W0.57Si0.43 have been produced by sputtering on GaAs(001) and annealed isochronally for 20 min in an arsenic overpressure at temperatures ranging from 500 to 950°C. Subsequently, solid state reaction products, microstructure and phase morphology were characterized by cross-section transmission electron microscopy and glancing-angle X-ray diffraction, whereas carrier concentration profiles, Schottky barrier heights and corresponding ideality factors were measured by capacitance-voltage and current-voltage techniques. Contacts consisting of W/GaAs are characterized by an equilibrium microstructure (β-W and/or α-W/GaAs) under all conditions, but corresponding electrical properties degrade substantially during annealing at 750°C and above. In contrast, W0.79Si0.21/GaAs and W0.67Si0.33/GaAs contacts are characterized by an amorphous structure after annealing up to 750°C, but revert to an equilibrium microstructure (α-W and W5Si3 on GaAs) during annealing at higher temperatures. Corresponding electrical properties degrade less severely during annealing at 750°C and above. Finally, W0.57Si0.43/GaAs contacts are characterized by an amorphous structure after annealing up to 500°C, but revert to an equilibrium microstructure (W5Si3 and WSi2 on GaAs) during annealing at higher temperatures. Corresponding electrical properties degrade during annealing at 500°C and above. In general, degradation of electrical properties is attributed to a combination of interdiffusion across and solid state reactions at the WSix-GaAs interface. A previously unreported cubic phase is also observed to form at the W0.57Si0.43-GaAs interface during annealing at 750°C and above. © 1991.