To investigate the effects of microstructure of the Schottky characteristics of WSix contacts to n-type GaAs, cross-sectional transmission electron microscopy, x-ray diffraction, and secondary-ion mass spectrometry have been used to study the interfacial and bulk film microstructures. The barrier heights and ideality factors of WSi0.1 and WSi0.6 contacts were obtained by forward current-voltage and capacitance-voltage measurements. These Schottky characteristics were found to be unrelated to the bulk film microstructure, but closely related to the interfacial microstructure at the WSix/GaAs interfaces. Both the WSi0.1/GaAs and WSi0.6/GaAs interface morphologies were observed to be stable and remain smooth during annealing at 800°C for 10 min, while a rough interface with W protrusions and Ga and As out-diffusion was observed in two-layer W/WSi0.6 contacts. The stability of the WSix interfacial microstructure is suggested to depend on both the chemical stability of the WSix films with GaAs and the intervening oxides between WSix and GaAs. Nontrivial amounts of W and Si were observed to diffuse from the WSi0.1 film into the GaAs substrate during annealing at 800°C for 10 min. Although these in-diffused impurities in the GaAs substrate do not seem to affect the Schottky characteristics after the 800°C annealing, they could be a potential problem in long-term stability. Of the three WSix film compositions, the single-layer WSi0.6 films were found to have the least W and Si in-diffusion and thus the best thermal stability.