The interfacial reactions between thin films of cobalt and germanium and (001)-oriented GaAs substrates in two configurations, Co/Ge/GaAs and Ge/Co/GaAs, were studied. The microstructure and phase formation, as analyzed by transmission electron microscopy, x-ray diffraction, and Auger electron spectroscopy, were correlated with the electrical properties of these contacts, as determined by current-voltage and capacitance-voltage measurements. At low temperatures, 250≤T<325 °C, the only reaction that was monitored was the formation of Co5Ge7 at the outer interface, while the GaAs substrate remained intact. The growth of Co5Ge7 was diffusion limited with an activation energy of ∼0.7 eV. At the temperature range of 325-400 °C for both metalizations epitaxial Co2GaAs was formed on top of the GaAs beneath the Co5Ge7 layer. For the Ge/Co/GaAs metalization this was accompanied (at 400 °C) by solid-phase epitaxial growth of Ge precipitates on the GaAs surface. Contacts produced in this annealing regime were rectifying with nearly ideal thermionic emission behavior. The Co2GaAs phase was unstable at higher temperatures (500-600 °C), and the reaction products were two ternary phases, with compositions of Co2GeGa and Co2GeAs. These compounds were spatially separated - the Co2GeGa layer on top of the Co 2GeAs phase. Contacts produced at the high-temperature regime (>400 °C) had very low effective barriers and on an n+ GaAs substrate became ohmic.