Interdiffusion and reaction in the GaAs/AuGe/Ni and GaAs/Ni/AuGe/Ni thin film systems have been investigated, using Auger Electron Spectroscopy depth profiling and Rutherford Backscattering Spectrometry. The roles of the individual metal layers were determined by studying the GaAs/AuGe, GaAs/Ni and GaAs/Ni/AuGe subsystems. It was found that the interfacial layer in the GaAs/Ni/AuGe/Ni contact scheme prevents Ge from diffusing away from the interface. In this way more efficient doping of the GaAe is possible. Current-Voltage measurements indicate that this layer structure forms a good ohmic contact to GaAs.