Recently, thermally stable, low resistance NiInW ohmic contacts to n-type GaAs have been developed using a conventional evaporation and lift-off technique and annealing the contacts by a rapid thermal annealing method. This contact material has great potential for use in GaAs integrated circuits. In the present paper, the microstructure of the NiInW contact material has been studied extensively by cross-sectional transmission electron microscopy. Special attention was paid to understanding the role of Ni in the NiInW contacts by analyzing samples prepared by different deposition sequences. In order to prepare the contacts with a large fractional coverage of InxGa 1-xAs phases at the metal/GaAs interface, which is essential to produce low resistance contacts, Ni must prevent In from spreading vertically into the GaAs substrate during the heating process. The formation of a uniform Ni2GaAs layer at the GaAs surface and suppression of In diffusion toward the GaAs by intermixing In with Ni at the initial stages of annealing were found to be critical to prepare such contacts with large fractional coverage of the InxGa1-xAs phases. In addition, Ni 2GaAs phases seem to remove native oxides at the GaAs substrate, which is also important in attaining good coverage by InxGa 1-xAs phases at the GaAs surface. Also, excess In was found to form high melting point Ni3In compounds which improved thermal stability.