In situ patterning and electrical characterization are used to study the electrical properties, reaction kinetics, and interface properties during reactions at Ni/n-GaAs interfaces. Ni contacts were formed in situ by deposition through a removable molybdenum shadow mask onto a GaAs(100) c(4X4) As-rich surface. Annealing at 300 degrees C resulted in Ni3GaAs formation. Subsequent exposure of the Ni3GaAs to an AS(4) flux at 350 degrees C resulted in the formation of NiAs at the surface and the epitaxial regrowth of GaAs at the NixGaAs/GaAs interface. The GaAs regrowth thickness and the regrown GaAs electrical properties were determined electrically by in situ capacitance-voltage and current-voltage measurements. An interlayer model was applied to explain the Schottky barrier height discrepancy between capacitance-voltage and current-voltage measurements for metal/GaAs contacts with regrown GaAs. The model predicts that the regrown GaAs interlayer is p type with a hole concentration of similar to 3 X 10(15) holes/cm(3).