Al-Ni-Ge ohmic contacts on n-GaAs were prepared by sequential vapor deposition and furnace annealing at 500 degrees C. The metallurgical properties of the contacts were studied by transmission electron microscopy. It was found that while Al-Ni-Ge as a whole is relatively stable against GaAs, extensive interfacial reactions readily occur within the contact layers, resulting in a very stable layered structure of the type Al3Ni/Ni-Ge/GaAs, with epsilon’-Ni5Ge3 being the major phase in the Ni-Ge layer. GaAs twins and Ni-As precipitates were found in a thin layer immediately below the metallization, suggesting that the ohmic behavior can be accounted for in terms of a GaAs regrowth mechanism.