In situ sequential depositions and reactions are used to control a reaction sequence first to consume GaAs and then to release GaAs from the reaction product. The first reaction involves annealing in situ deposited Ni on molecular beam epitaxially grown GaAs (100) at 300 degrees C to form a Ni3GaAs/GaAs structure. Exposure of this structure to As-4 results in a reaction which consumes the Ni3GaAs by the layer-by-layer formation of NiAs at the surface and epitaxially regrown GaAs at the Ni3GaAs/GaAs interface. The NiAs formation and GaAs regrowth are controlled by the As-4 flux. Ni diffusion dominates both the Ni3GaAs formation and decomposition mechanisms. Reflection high-energy electron diffraction, Rutherford backscattering, x-ray diffraction, and transmission electron microscopy data are used to confirm the phase formation and reaction sequences.