In situ electrical characterization is used to study the interface properties and the contact penetration during reactions at metal/semiconductor interfaces. Ni contacts were formed in situ by deposition through a removable molybdenum shadow mask on molecular beam epitaxy-grown n-type GaAs(100) c(4x4) As-rich surfaces. Annealing at 300 degrees C resulted in NixGaAs (x approximate to 3) formation. Subsequent exposure of the NixGaAs 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 Ni3GaAs/GaAs interface. The Schottky barrier height (phi(bn) = 0.68 V, as deposited) increased with NixGaAs formation (phi(bn)=0.87 V) and decreased slightly with subsequent As-4 exposure (phi(bn) = 0.85 V). A thin buried n(+) marker layer was used to determine changes in the metal/semiconductor interface position from in situ capacitance-voltage measurements. The marker-layer movement demonstrated consumption and subsequent regrowth of GaAs beneath the contact. The ideality factor obtained from current-voltage measurements for the contacts on regrown GaAs was less than or equal to 1.11, which is indicative of the high electrical quality of the regrown GaAs.