Interfacial reactions between cobalt thin films and single-crystal GaAs substrates have been studied by means of Auger electron spectroscopy, transmission electron microscopy, and x-ray diffraction. The interaction starts at ∼325°C by the formation of a ternary phase, most probably Co 2GaAs, which grows highly oriented with respect to the (001) substrate, with a lattice mismatch of ∼-10%. The reaction kinetics has been studied and found to be diffusion controlled with an activation energy of 0.7±0.1 eV. Cobalt was determined as the dominant diffusion species. The oriented ternary phase co-exists with randomly oriented CoGa and CoAs at the temperature range of 325-500°C, while at higher temperatures only the binary compounds prevail.