The metallurgical examination of solid-state reaction between nickel thin films and single-crystal GaAs substrates and the resultant electrical properties of the contacts are reported. Annealing at 100-300°C in forming gas led to formation of a metastable hexagonal phase Ni2GaAs which was stabilized due to its epitaxial growth on (001) and (111) GaAs substrates, as follows: (101̄1)Ni2GaAs∥(001)GaAs and (0001)Ni 2GaAs∥(111)GaAs. Nickel atoms were found to be the dominant diffusing species during the ternary phase growth. Ni2GaAs is stable on (111)GaAs up to at least 600°C, compared to 350°C on (001)GaAs. The larger stability on (111) is explained by the better epitaxial match found in this case. Reaction on (001)GaAs in the temperature range of 350-550°C resulted in decomposition of Ni2GaAs by NiAs precipitation. After annealing at 600°C the reacted film was composed of two phases: NiGa and NiAs. The electrical properties of the contacts were correlated to the phase interfacing the substrate. The Ni2GaAs formed rectifying contact with a barrier height of 0.84 eV, whereas when NiGa and NiAs were at the interface an ohmic behavior was observed.